It has to be a mental issue, something in his head kicks in, self doubt, call it whatever but it certainly is nothing to do with his talent, the doubles are twice the size of the trebles and he has absolutely no trouble at all hitting those.
When he first came on the PDC scene he shot straight to the top, earned his spot in the Premier League and won it, then I think he maybe had a match or two where he missed a lot of doubles, the more doubles he missed the more doubt set in, and now, as he has lost (and almost lost) many matches due to his failures on the doubles the problem worsens, I bet it is hell for him, every time he gets to the double something in his head will remind him of all those misses, severely multiplying the pressure and again more often than not cause him to blow it.
Until he gets his head right the problem will not go away IMO, I bet he practices doubles for hours on end, and I bet he hits 75-80% of them in practice but once out on the stage the gremlins are at work.
I hope he gets it right soon, I like the bloke and there is no doubt scoring wise he can beat anyone on the planet.