Some people just cant play and sing at the same time...BB King is a very good example...he doesn't even play rhythm while he's singing...his lead lines just come in between the vocals...!!! Not saying everyone is like this but it does happen...
I don't know what level of player you are but I would guess that you still have to think about what you are playing as you play it...in my experience this sort of syncs the brain and the playing and the singing into one big lump. My advice would be to learn a song so well on the acoustic that you just don't have to think about what you are playing anymore...this frees up the brain to concentrate on the singing...try something really simple like knocking on heavens door...just 3 chords...another bit of advice is to learn the chords until you can play the song with your eyes closed...If you can play a song without looking at either hand then you are playing from pure muscle memory and not using your brain...this should leave your brain free to figure out the singing...and disconnect it from the rhythm you're playing...it's not easy to do two things at once but it gets easier if you can do one of them in your sleep...!!!
Hope this helps...
Craig...