Slave can still be bad. I put a used slave in my truck that was only a year old. It failed within a week, the fact that it's only a year old is no gaurantee of anything.
Double clutching not helping, along with the lack of grinding tells me that it's unlikely to be bad synchros, but still not impossible
The fact that it only happens at a stop is telling me that there is a release issue. For whatever reason your clutch is not releasing fully. The fact that you can get it in easily doesn't detract from that, it actually supports the idea of partial release.
The gear teeth have to line up with the recesses on the other shaft to go into gear. It's hard to get it into gear at a stop because the output shaft is stopped, the input shaft is still partially attached to the engine, and it's light enough that it will move even if the clutch is only partially engaged. It's easier for you to get into gear while moving because both shafts are moving now so the teeth line up right on their own.
It goes in when you force it at a stop because if you put enough pressure on it the teeth will catch, but its harder because one shaft is moving and one it stopped.
You either have a hydraulic problem somewhere, or your pressure plate is no good.
If you sit at a stop with the clutch down A LOT there is a chance that the tension spring that sits between the TOB and the salve has gone bad already, but that is HIGHLY unlikely.