Strange behavior with Delta

Hello,

After many months with no use of this DELTA printer, I've decided to use it again.
But strangely after "home" when I do G1 Z100, the Y carriage is lower than two others and the difference is the same each time. Originaly, the steps/mm for all motors was to 160 and now I have to setup 153 for Y motor if I want all carriages go down with the same distance !! All three motors are the same, 0.9°/step. If someone has an idea ?

you sure that all 3 Toothed pulleys have the same number of teeth and check that they are all tight on the motor shafts.

It may also help if you post your Config.g file?

Doug