Why is it when you glue shells or sticks they fall off after a few wee

Good day. I am from Namibia so there is a bog time difference.
I love to use my glue gun (big and small ones), but I discovered that the glue only lasts for a few weeks or say 6 months (it depends) the longest then it started to come lose.
Is it due to our hot dry weather or is it just weak glue?
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