I doubt that it is because your hair has sustained some damage from overprocessing.
I really do suspect that if you are installing them yourself and they keep regularly following out that it probably *does* have to do with your installation technique!
Some common flaws I see with installs are the following:
-Weaving the hair in too thickly. If you take the hair and weave it in in thick little chunks, you are likely to have those extensions slip out. They have an easier time weaseling their way out if you just weave them in. This problem often goes hand-in-hand with using too large of a crochet hook for your install or using a latch hook for your install. These are not good tools to install extensions. If you are using a 1mm or larger crochet hook, you will not have a good install. Period.
See what I mean in this fancy, hand-drawn picture!
The next issue I see is that there isn’t enough overlap between the extension and the lock you’re installing it on. You need about 2-3 inches of overlap between your hair and the extension for the extension to stick:
The best way to install extensions is with a 0.5mm crochet hook (can be purchased on amazon here- http://www.amazon.com/Clover-Touch-Steel-Crochet-0-50mm/dp/B0013AG9OC/ref=sr_1_1?s=arts-crafts&ie=UTF8&qid=1394157658&sr=1-1&keywords=Clover+Soft+Touch+Crochet+Hooks%2C+Size+14 ), and a substantial overlap of your hair and the extension.
Here’s another fancy drawing to show you what I mean:
Finally, my video demonstrating how I installed my extensions can be seen here: http://www.youtube.com/watch?v=Hnfinze-_co (the installation starts at about 15:00 )
So, I hope this helped you troubleshoot your extensions.
Really, if they come loose, it is almost surely your technique!
Another issue is that you should not install them on new locks. Unless you have a lot of experience, oftentimes the extensions and new locks will be too loose and come undone.