You may need to make your fingers stronger since it takes alittle more pressure to push down on the strings enough to produce a solid sound. The sissy sound is usually caused from this, and it probably gets worse if your using your pinky finger. Just because it's the weakest, but just keep practicing. Your fingers should get stronger, if you still have trouble after some time. Get yourself a one of those finger-strengtheners for guitar, that shouldend the problem indefinitely.

The idea is like taking a thin long wooden board and then laying 2 concrete blocks under each end of the board. Then standing in the middle of the board. The board will automatically bend down because of your weight. But if you bring the concrete blocks closer together, the board bends down less. Even though the weight on the board is the same. Well it's the same thing with the guitar. The closer the frets the less the string bends between them. Especially when there as close as the 21st and 22nd fret.