What glue to use on wall decals that are falling off?

[jemshkoj]

I have wall decals that have been up for 2 years, but are starting to unstick due to the weather changes. Is there a glue that can be used to re-stick them? I know of post it glue, but is it safe on painted walls?
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calendar wall decals

 

[Karis]

What sort of wall decals are you talking about?

What are they made of?

Are they outside & exposed to the weather?

What sort of wall are they stuck to?

What did you stick them on with originally?

I doubt post it glue will be of much use!