I May be wrong (I frequently am
)...
99% of the time TC give a release to Solent (incBournemouth traffic) with no issues. However, on occasion we have to say "no", or "not yet", or "I'll call you back".
Because airfields get so used to us 'always' giving release, they line an aircraft up in anticipation of it being able to go immediately, then are slightly embarrassed on the few occasions it doesn't happen. Wouldn't be the first time, and doubt it'll be the last.
Equally, it is possible that Solent won't give release unless aircraft is ready to go.
I can't remember if we have an official Clearance Expiry Time on Solent (inc HH traffic) but certainly the guys and gals at Solent Radar (who incidentally do a great job IMHO) know that if we give a release we expect to see the aircraft fairly pronto. In addition, if the departure is Northbound at rush hour in the morning it may be subject to a Miles in trail restriction and be having to be fitted in with Southampton Northbounds.
Could be any number of reasons, also aluded to on here.
Best bet is def to give the tower a call and go visit and ask; ATCO's are not all pilot eating monsters
and you will get the answers you are looking for, and maybe find out how you can help with the smooth running of things too...
Cheers,
Northerner
"Keep smiling, it makes people wonder what you're up to..."