Actually the way it was explained back in Uncanny X-Men (1st Series) 138 and 144 published in the early 80's - Scott Summers, AKA Cyclops, does not have full control over his powers because of a head injury he suffered as a child, when his mother pushed he and Alex (Scott's younger brother and later the X-Man Havok) out of an airplane in order to save their lives:
Cyclops's head injury caused an inability to fully control his optic beam as explained here:
However a retcon, by Joss Whedon in the pages of Astonishing X-Men, suggests that Cyke is in fact the one who does not allow himself full control over his mutant powers:
But as Emma Frost was under the subliminal control of Cassandra Nova when she was messing about in Scott’s brain, so it isn't 100% clear that this is actually true. The control he gained from that incident was temporary and in the end he required his ruby quartz lens to again gain some control over his optic blast:
As is the nature with the X-Men convolution abounds, because if those two options weren't enough for you way back in X-Factor (1st Series) 39, circa 1988, this was revealed:
So the reality is that it could be that Mr Sinister, Scott himself or even that ole head injury that does it... But, if you don't mind a little speculation, one would think if it was either of the first two Professor X, or even Phoenix, would have be able to help due to their vast mental powers?!