What causes loca uv glue to crack and split?

Please see image attached.

I want to know what is causing this to happen? I do not think it is the glue as I have used half without problem but the last few I have done has started to crack.

Posible causes.

Glue storred in a cold place

Not enough glue

Too much glue?

In the past I have done loca repairs and then placed the phone in bags after and a day later all the glue started to come away from the edges, my guess that it was caused from sweating in the bag.

But what could be causing this? If I had to guess my self I would say cold storage and not enough glue?

Any help would be great.