First thing to try is screwing in the cable tension adjuster about a 1/2 turn (or you may need to turn more so try a little at a time). This effectively gives the cable more slack allowing it to release the derailleur farther downward when shifting. Could also be something like the cable or derailleur is sticking or something else but try this first. Also if you find after the adjustment that shifting to the smaller cogs now works great but shifting up has become worse turn the adjuster 1/4 turn out and check again.