Upper hardware top keeps coming off glass tank

Firstly let me say that I love my Aspire premium kit - I use it everyday.
A small problem I have been having lately is the upper hardware top falls of the tank and I have to keep pushing it back on.
When the top falls off I can see the rubber seal on the tank and I just push it back on and it holds for a while before falling off again.

I can still use the ecig and it doesn't lose any vapor, it is just annoying that I have to keep pushing the upper hardware top back on..

Any suggestions on how to fix this? Has anyone else experienced this?

Thanks.
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