Can't focus eyepiece

Hello everyone,

i was lucky to get my hands on a used 4,7mm tele vue ethos for an "OK" price. I was eager to try it out today and the weather was good, however I can't get it to focus. I have a skywatcher 8 inch dobson. I use the eyepiece with the 2inch adapter. I have the 2 inch connector in the dob, and the other eyepieces I usually use work fine and I was able to focus them, but for some reason I can't get the 4.7mm to focus.

Tried to focus by targeting the moon, and when i approach the maximum how far I can move the focuser out, I can see the image starts to get sharper, but before the image gets really sharp, I hit the end and I cant move further out. its like the focuser would need to be some mm longer.

Any Idea what I could be doing wrong? where I live I sadly do not have a local astro community to ask..