The 12X7 is most likely the cause of that. A 7 pitch prop is mainly for speed not pull and will not give you much vertical. You want to go to a 6 pitch or even a 5 pitch. Most likely an APC 13X5, maybe a 12X5 or even a 13X4. The 13X4 will give you your best pull vertically. You will lose some straight line speed but it will climb pretty well.

Think of it this way. Larger diameter lower pitch equals pulling power, lower diameter higher pitch equals speed. I'd bet you are having a hard time slowing it down for landing

That particulr engine like to tach about 9500 to 10,000 so you prop accordingly.