In Battle of Gods Gohan still has his mystic form, yet in Fukkatsu no F and in Dragon Ball Super he lost it. I believe it was in Fukkatsu no F that he says he can barely turn into super saiyan. Was it ever explained why he lost his mystic form?
Yes it was explained during this arc (I don't know the exact episode as I watched it when it was airing) that he lost control over his powers due to him becoming rusty. This resulted in him starting to train again with Picolo to regain the powers he once had, including his mystic form.
Gohan decided in the past to focus more on his studies rather than being a martial artist. This made him become rusty... very rusty, rusty enough to barely transform into a super saiyan. His mystic form is beyond that of a super saiyan. I am not sure how strong it actually is, but it is definitely more powerful than his previous super saiyan forms. It might be as good as ssj3, but that is just my personal speculation.
Since he focused so much on his studies he was able to achieve a lot things as for getting job offers and such (as seen in previous episodes). He completely neglected his trainings due to this and became very rusty as Picolo mentioned a couple of times throughout dbs.
I don't recall him turning into his mystic form though during the Battle of the Gods.
But to get back on your question, according to the timeline of dragonball it had been 5 years between Buu saga and DBS. During Buu saga he wasn't training much anymore and in those 5 years he didn't train at all (mainly due to Chichi not wanting him to turn out like his father). The effect of this probably lowered his powerlevel significantly which made him for him almost impossible to turn into ssj without training again.