My slant on "retention" is that it's not as great a problem as it used to be. By that I do not think it is insignificant either or that there isn't room for improvement.
Truth is lots of people join only intending to do around 5 years, so you can hardly be surprised if they do as intended. You don't have to do the full term.
I recently had someone's Mother draining down all over me because her "little darling" was doing a "boring job". The same person said that they needed entertaining, excitement, thrills & stimulation.
Fair call, but I thought it was a job we did, not a substitute for a Gamecube. All those things happen, but not simultaneously & similarly someone has to clean the bog sooner or later, 'cos Mum isn't there to flush it for you.
Currently in my region about 15-20% of our annual input into the RN/RM comprises re-entries (former service personnel) at the moment. Granted, this fluctuates, but generally it rises when the civilian workplace has less to offer.
Of course there are may reasons for rejoining, not least because you can up to the age of 37, but the majority admit to making a mistake when they left first time.
Moral to the story: no matter how much you may dislike the job you are currently doing, don't burn your bridges when you leave.