After a few dodgy results I decided to go back to basics with Top Spin school and as the computer tells you what buttons you were pressing I discovered I am not actually hitting the trigger buttons on my ground shots.
On the serve I have no problem but I am either pressing too lightly on R2 and L2 or pressing them to late (I think it's the first one). I've never noticed before because with high power it's hard to tell but I can't believe all the times I though I risked, 95% of the time I probably wasn't.