Many of my former colleagues have returned as FTRS, two of which surprised me enormously as they where hugely vocal about their reasons for PVR'ing, yet within 12 months had returned, unable to cope with the need to "be someone", and unable to earn the respect they demanded in civvy street that wasn't instantly available because of what was on their shoulders. Others have moved from regular to FTRS seamlessly because they were approaching the end of their career as a regular and didn't feel it was time to leave the comfort blanket the RAF offers. I don't know of anyone who has left the RAF, successfully carved out a new career and then left that after 3+ years to return to an FTRS role although I'm sure there are some, and I'd be interested to know why someone would leave a successful career in civvy street for an FTRS contract.
As for recruiting new entrants - as others have said, you can only interview those who apply. So many kids go to uni these days as a matter of course, then the pool of 18-21 year old applicants has already had most of the cream taken off. I've talked to several of my younger, non-uni graduate colleagues at work about military life, and not one of them has said that its something that they would be interested in doing. Neither of my kids have any desire to do it. The recruitment pool is very small indeed.