This assumes two travelers between the switches.

Assign each switch a number.
With switch #1 & switch #2 down, light is on or off?
With #1 up & #2 down, light is on or off?
With #1 down and #2 up, light is on or off?
With #1 & #2 up, light is on or off?

In principle, with the answers filled in, the problem(s) can be pinpointed.

Test cases welcome. . .