Adhesive for iPad2 not so strong, what is best method

I purchased adhesive off the internet and it seems to be the popular kind that everyone is selling/buying.

I changed battery in my ipad 2 and used this adhesive to put screen back. all seemed well and nice and the screen seems fine after a few days...

Buttttt....I go to open up the ipad 2 again, and the screen came off way to easily, adhesive was not so strong after all. The original apple adhesive was difficult, but not the one I purchased. What can I do to make sure future screens are in more securely than this.

Thanks