Slots are given based on the EOBT on the FPL your ops have filed, if they forget to delay it, it's not CFMUs fault!
It might be frustrating, but I have experienced (several times, simmilar situations) that a flight lands late, and I give heads up regarding a CTOT on the outbound leg ca. 25 min later.
I offer to delay the FPL for them (not my job, but quiet afternoon), and is told not to, because they will make the slot. (!)
1:30 later, they are stil at the hold, engines running because they missed the slot, and got bumped by CFMU! (And yes, I was ready to give them the 5/10min leeway, but they missed that too)