Without a more detailed description, it's hard to say exactly what's wrong. But large focus variations across the screen are rare without an accompanying symptom, such as incorrect raster size (or symmetry problems), or a brightness problem. If the brightness is set too high, focus variations will be exacerbated.
If you cannot increase brightness to normal values without focus degrading massively, that's usually a sign of a worn-out CRT gun. However, it can also be caused by a too-low voltage on the 12V supply (the CRT filament power comes from the +12V supply). Measure it and correct if necessary.
If the raster is too large, that is generally a sign that the HV supply has a problem and the voltage is too low. That, in turn, will usually be associated with a dim raster and poor focus.
If you could post a pic, or provide a more detailed description, that would help us narrow things down a bit.