500N, true, no loss of entitlement perhaps.
Only so much leave (RAF) can be carried forward in any one year. Units have leave plans and leave embargoes. If you lost your two weeks in the summer school holidays there is slim chance of regaining it this summer.
With leave plans thee may well be available slots later in the year and indeed the holidays may be cheaper but for those with school age children . . .
The leave year finishes at the end of March. You may just run out of time or run in to Ops or pre-Ops work-ups. Autumn is the exercise period and so on.
You can simply run out of year in which to take leave. That said, in almost 40 years I always got my full entitlement.