Some of the time giving you a short cut will result in us having to make phone calls and coordinate with the military/other sectors/other units* delete as appropriate and we're not always in the position to be have time to do that. It can take you outside controlled airspace which isn't so much of a problem when we're quiet and we can keep an eye out for the 7000 puddle jumpers
Other times, one sector is happy to offer it, but the receiving sector is busy and doesn't want traffic coming from a non-standard direction. when you get busy, non standard stuff is the first thing that goes out of the window - it's easier to cope when everyone is doing what they're supposed to do.
From a personal opinion, if I'm in a position to offer a short cut I will. In terms of asking for one, just use a bit of common sense. If it sounds busy on the RT, then it's unlikely you'll get one, you're also a lot less likely to get one when the military are active especially if you're inbound to Dublin from the CPT direction.