NEW YORK (TheStreet) -- This year is shaping up to be among the biggest ever for mergers and acquisitions, according to Dealogic. Global M&A volume was $2.28 trillion in the first half of 2015, the second highest on record.

Still, not every company that tries to sell finds a buyer. In fact, a lot of companies fail to sell, said a technology banker. The reality is some assets are difficult to sell, and when companies ultimately don't find buyers, management isn't surprised, the banker said.

A second banker said it's hard for large, public companies to sell, because the pool of buyers shrinks with price and number of products.

Sellers may want more than buyers are willing to pay. Companies deteriorate too quickly for some buyers to consider. Governments sometimes won't let deals transact.

Here are a half-dozen technology companies that have sat on the auction block for years:

Image placeholder title

T-Mobile USA(TMUS) - Get Report -- T-Mobile has had plenty of interested buyers, but the U.S. wants it to stay the fourth-largest phone company in the country.

It's attractive to wireless carriers who want to gain scale or remove a competitor and to TV companies who want to compete with wireless carriers, said Colby Synesael, an analyst at Cowen.

It's had numerous offers. In March 2011, AT&T(T) - Get Report announced plans to buy T-Mobile for $39 billion. Nine months later, AT&T decided to pay a $7 billion breakup fee rather than fight U.S. antitrust regulators. A few years later, Softbank (SFTBY) attempted to persuade regulators to let its portfolio company Sprint(S) - Get Report buy T-Mobile but also gave up. The latest rumor is that Dish(DISH) - Get Report wants to acquire T-Mobile.

A spokesperson for T-Mobile declined to comment.

Image placeholder title

BlackBerry (BBRY) -- BlackBerry was once THE smartphone maker. But in 2007, Apple(AAPL) - Get Report introduced the iPhone.

The iPhone and handsets with touchscreens were so slick and easy to use, they unseated the cumbersome BlackBerry. Rumors that BlackBerry could sell began to surface around 2009. If BlackBerry were attractive, it would have been for its brand, intellectual property, and security and maintenance division, said Andrew Uerkwitz, an analyst at Oppenheimer.

BlackBerry did not return requests for comment.

Image placeholder title

Juniper Networks(JNPR) - Get Report -- If you subscribe to the theory that any company with Elliott Management as an activist investor is for sale, then Juniper's been for sale for almost two years, the first banker said. Elliott, which helped orchestrate the takeovers BMC, Compuware and Informatica, began agitating at Juniper in January 2014.

Juniper is a tricky company to sell, said Alex Henderson, an analyst at Needham. It's a big networking hardware manufacturer at a time when the cloud is reducing the need for those kids of products. Even so, Juniper has a "great footprint" among telecom carrier customers, said Erik Suppiger, an analyst at JMP Group.

A spokesperson for Juniper declined comment.

Image placeholder title

Computer Sciences Corp (CSC) -- Rumors of a CSC sale are 10 years old, the first banker said. Speculation first appeared in 2006, and every now and then, publications post updates. The problem with a sale is that different groups of buyers would take interest in its commercial and government businesses, and its commercial business is also a mix of more and less attractive assets, the banker said.

A spokesperson for CSC declined comment.

In May, CSC said it would split into two companies: commercial IT and government IT. The split makes each half more "digestible" to potential buyers, said Frank Atkins, an analyst at SunTrust. If an acquirer wants parts of CSC, it would be for a customer set of many Fortune 500 companies and expertise in insurance and some government segments, said James Friedman, an analyst at Susquehanna Financial. Susquehanna has a positive rating on CSC and a price target of $67.66 a share. SunTrust has a neutral rating and a price target of $78 a share on CSC.

Image placeholder title

Rackspace (RAX) -- Rackspace said it had retained Morgan Stanley(MS) - Get Report to explore options more than a year ago. A public sale is always risky, and Rackspace had to explain to investors that it would remain independent a few months later. According to reports, buyer interest was modest, and nobody wanted to meet its asking price.

Rackspace didn't return phone calls.

Still, Rackspace sells IT services as an added benefit to using its cloud, which may attract buyers at some point, Cowen's Synesael said. Cowen has an outperform rating on Rackspace and a price target of $74 a share.

Image placeholder title

Yahoo! (YHOO)  -- If you subscribe to the theory that once a company is for sale, it's always for sale, then Yahoo! is perennially for sale, the first banker said. In 2008, Microsoft made an unsolicited bid to acquire the company. But a hostile bid always contains a level of, well, hostility, and the companies failed to agree on a price. Meanwhile, Yahoo! explored possible deals with a host of other companies, according to reports.

Yahoo! didn't return phone calls.

If a buyer is interested in Yahoo!'s core search business, rather than its stakes in Alibaba(BABA) - Get Report and Yahoo! Japan, it's attractive for its "huge" user base and potential growth in video, mobile and social web, said Victor Anthony, analyst at Axiom Capital.