If the insert in the wood that holds the wheel has somehow been damaged, the stroller wheel might fall out. You can try using epoxy to glue the stem part of the caster into the insert or you can replace the entire wood/caster assembly.
If you have an older model stroller where the wood parts fit a specific leg, you can still use the current model wood/caster assembly. In other words, you can mix & match the wood/caster assemblies.