Focal issue with ZenithStar 110mm

Hello everyone,
I just need your help for a small problem. I recently bought a used ZenithStar 110mm, and I don't know why, I am not able to make focus with my two 10 and 20 mm eyepieces. I put extender behind the Zenith and it seems that focal point is still far behind. Do you have any idea where could it come from. Is it possible that focal lenght move during shipping or does it come from my eyepieces ?
Great thanks for your help.
Arnaud

Arnaud:

The focal length has probably not changed.

Are you using a diagonal and if so what type are you using? Most refractors like the ZenithStar are designed to be used with a 2 inch star diagonal, these add a significant amount to the optical path length so if you are using a 1.25 inch or no diagonal at all, you will need to add quite a bit to reach focus.

Jon