If the screws are hard to insert into the enclosure bottom, don't force them but look to see if the keys mount rail is in the way. It may have come loose from its aligners.
A note is stuck on? The key mounting screw(s) may be too tight. With the Scout turned on, try loosening the screw until the note turns off.
A note needs to be pushed really far down to play? Or it turns off when pressing another near it? The key mounting screw(s) may be too loose. Try tightening!
The speaker/battery wires can sometimes get in the way of keys, making them difficult to actuate. Pop the enclosure apart and see if you can make them lay flatter.
Most other key problems are from some obstruction between the key piece and the enclosure. Read through the "Assemble top" section again and verify everything looks correct.
If the volume knob is hard to press onto its pot shaft, don't force it -- that could permanently damage the potentiometer. Try filing the inside of the knob's shaft cavity. If printing at home, make sure you've got the most recent STL models.