You need a new one. Simple as that. Right now, I know of FOUR people who have the exact same problem as you. It's a bad design on the plastic G36s from TM, JG, etc.
That's why I have mine replaced with a full metal inner assembly.
I don't know how you are searching but I found like 3 on my first google search. They are plastic - that's true. The metal ones are rare and expensive.
http://www.airsoftgi.com/product_inf...oducts_id=4814
or:
http://www.airsoftpost.com/preorder-...t-p-28847.html
or just this:
http://hotspotairsoft.com/catalog/pr...oducts_id=2746