Every sim has a "prim reserve" to allow for temporary prims and vehicles, which are not supposed to be counted against the prim limits of individual parcels, because they're temporary or (normally) just passing through. In the past year, however, prim limits on parcels have sometimes been applied against vehicles, which has led to vehicles being prevented entry to parcels when they should be allowed in. This may have been done as a side effect of fixing a problem with temporary items, or with the wrong items being returned when someone stands up from a vehicle. Whatever the reason, it's causing big problems for flight and needs to be fixed. At least one case where this happens is when the vehicle is crossing the parcel over a sim border, but it doesn't seem to be the only case.