There is no need for them to be full time yet or they would be. There is a reason (mostly financial I'm thinking) they are not. For years we've heard about the royal family trimming the working royals down to smaller numbers. It seems to be that is exactly what they have done.
Charles will likely be King in the next 5 to 10 years at most. When William is the heir he will certainly be full time.
For now the Queen (and probably Charles) determine who's doing what when.
LaRae
Thank you for this reminder Pranter. Just like in any other "Firm" the decisions made come from the top and have to take budgets into account. At this point in time the BRF are the largest group of "working" royals who undertake official engagements on behalf of the monarch. They're also unique in that there are 3 generations of adults in that group. In most constitutional monarchies it is limited to just two generations often times because of their nation's constitution limits the size.
The press chooses to focus the majority of their attention to just a few royals and are not pleased that they're on the part time list. So to sell a few more copies we've seen an increase in articles that are not always based on facts.