I've noticed that every product I've bought in the past year with a zip-lock seal is destroyed with common use. I actually think the zip-lock itself has gotten stronger. The bag rips before the seal opens on half the bags now and whenever I try to opens bags I had no issues with before, I find myself stretching/warping the plastic before the seal eventually opens.
It's pretty frustrating and I am seeing it across many products. Cheese bags, storage bags, snacks etc...
There is actually a trick to open them. Just slide the two halfs of the zip along each other and (between your fingers) the ends will pop open. This is a trick I use in my retail job almost every day.