There seems to be a stock set of reasons why DVD releases are held up, but we haven't made a list of these yet, so I have decided to start one.
Clearance issues: This often means music used in a show will cost a lot of money for the DVD producers to use again as the music is only bought for the initial run of the show and maybe repeats. A point in case; the last episode of 'Citizen Smith' used the Beatles' 'Here Comes the Sun' extensively, so it it was more cost effective to substitute the soundtrack for another piece.
Expensive negotiations: Cue Aaron on this one, as he will be able to explain this one better. What it sounds like is that it sometimes costs a lot of money to even talk about a show with the BBC.
Artiste's discretion: Quite simply, a star of the show doesn't want it released on DVD. Martin Shaw blocked the release of 'The Professionals' for a while. This is the reason why 'Rentaghost' stalled at the first series; one of the artistes didn't want the rest to appear on DVD.
Cost Effective: Similar to clearance issues, but this time it didn't do make much when the first couple of series were released, therefore the production company will deem it to be not worth the money to continue releasing the series. I think '2.4' and 'BoaF' fall into this catagory.
Forgotten: It could simply be that it's been forgotten and it wouldn't even have been thought of for release.
I think this pretty much covers it?