I have Q-rings (50/34) on an otherwise stock 2010 Cervelo P2 with Shimano Ultegra 6700 parts. When downshifting from the big to small ring, the chain sometimes slips between the rings and spins freely. This happen more often when in the smaller cogs. I have to switch to the middle cogs and shift up and down again to get the chain on the small ring. Is there anything I can adjust to remedy this issue?

Are the chain or chain rings maybe worn out?

Check the inner ring isnt on back to front. Easy to do and it moves it a mm or so inboard which now leaves a bigger gap between the rings than normal. Can still cycle and change gear, but will get the accasional drop

Its very easy done (Guess how I know!)