I have this problem occasionally too. When I played with a counter style player I was always able to execute this shot. Now that I play with power base liner at 40% top spin, the shot only works 1/3 of the time it seems. Sometimes it goes long and wide when i try it as well. I know it's not my execution of the shot, (I've been playing since 2005 and play on Elite controls on Master 6 in World Tour) and have every other shot mastered I used to play on Incredible but it's not realistic to the pro-tour so I lowered it. This one shot just seems to not work for me on a regular basis which sucks because I love to use it to pull my opponent out wide or as passing shots when they are at the net.
Like you said; it works 100% of the time for me on backhand short angle top spin, but not regularly on the forehand... So confusing