"First of all, the handshake deal that happens on the show isn't anything binding. Filming for each season is split over the start of the summer and the start of the fall. After each round, the Sharks and their teams do due diligence on each of the companies to ensure that everything within the company is how it was represented in the pitch room.
There are times when the numbers check out, but the entrepreneurs decide they no longer want to make a deal, such as when the founders of evREwares decided they weren't actually willing to sell 100% of their struggling company to Cuban for $200,000 as they agreed to in season six.
Ultimately, said Cuban's fellow Shark Daymond John, about 80% of season seven's deals made on camera closed, which is up from about 60% to 70% of past seasons."
http://www.businessinsider.com/what-happens-after-closing-sh...
Edit: formatting