Problem with changing to the inner chain ring.

AJCIOW

I have an Ultegra SL chainset with 105 10 sp groupset. I have a problem if i downshift from the 53 to the 39. The chain does not index onto the teeth of the 39 and the crank spins. It only does this if the chain is on the smaller rear cogs, from about the 19t or 17t (half way). I had a compact 105 on before and this was not a problem. It it just my technique, do i need to be on the 19t + to change down?