Skin diving ditch and recovery is a skill that most candidates have a problem. It is really not that difficult but most make a mistake in how much breath hold they need to complete the skill and they over inhale. If you think about it, how long does it take to put on two fins and a mask on? Not long. Ten seconds maybe. That is all the breathhold you need to complete the skill. It does not take a full, full breath. That much air in the lungs will make you very buoyant causing you to fight to stay down and therefore shortening breath hold time. The more relaxed you are the longer you will be stay down. Try this.
Go to the bottom of the pool. Do nothing, just sit and mentally run through the skill. Find out how much do you need to inhale to complete the skill, how much air do you actually need. I am sure not a full lung. First try removing fins and mask. Place them neatly together. If you have floating fins, place a weight on top, it?s allowable. Surface and relax. Go back down, one fin on, then another. Put on the mask LAST and clear. Do the actual exhalation just like clearing a mask. The reason why the mask last is by the time you put the mask on, by clearing it you will exhale some CO2 which will give you few more moments of breath hold time. Practice this mental exercise until your are comfortable with time. Do not push, do not extend yourself till you fell hunger to breathe. Only when you can run through the skill mentally, then start practicing the actual skill.
I hope it helps. Works every time.