the pendulum mechanism may not be level even though the wooden case is. Try raising one corner at a time a little bit with some cardboard and swing the pendulum. If the tick-tock sound is not even and equal, lower that corner and raise the next. Do this until you get an even tick-tock sound. Then you can permentaly level it in that position with the levelers on the bottom.
If that doesn't work, or you have absolutely no sense of rhythm, then call a reliable Horologist (clock repair person) - and have them make a house call and get it fixed. A working clock will give you great pleasure.