Are you sure about that? If the reason is that a cache had its D/T changed (which seems likely) then Project-GC doesn't get notified about that so it can take a rather long time before we notice if the cache doesn't get a new log (a month at least). If the cache also happens to be archived, it can take even longer. It will eventually happen, but you may need to wait a bit.
If you know which cache is the problem you can request a refresh for the relevant GC code via Support -> Self-support to speed this process up.