Problem focusing with higher power eyepiece

[moosestang]

I recently acquired an Orion StarBlast 4.5 EQ.  I can focus on objects with the 15mm eyepiece with the focuser in about the middle of it's travel.  But if I switch to the 6mm eyepiece, I have to move the focuser down to the bottom of it's travel where it is still not in good focus.

I posed this question in another forum, and it was suggested that the primary mirror might be too far back.  I did move it as far forward as I could using the adjustment screws, and it did help some.  But the focuser still seems to want to be lower to achieve focus.

I was also told by another user who has the same telescope that he can achieve focus with either eyepiece with only a slight adjustment  of the focuser, not the extreme travel I seem to require.  So it seems something is not right with my setup, but I don't know what.

Collimation seems ok.  Looking for suggestions on what to try next.

[Ricochet]

If another user can focus the 6mm eyepiece near the 15mm eyepiece focus position, perhaps there is a fault with your eyepiece. I would try some other eyepieces and see if those focus properly. 

[wookie1965]

Does your eyepiece go straight in the focuser or in adapter. 

[moosestang]

1 hour ago, Ricochet said:

If another user can focus the 6mm eyepiece near the 15mm eyepiece focus position, perhaps there is a fault with your eyepiece. I would try some other eyepieces and see if those focus properly. 

I've also tried using the 15mm with a 2x barlow.  This will focus ok, but almost all the way to the bottom of the focuser travel.  So there is a trend: higher magnification requires the focuser to be lower.

59 minutes ago, wookie1965 said:

Does your eyepiece go straight in the focuser or in adapter. 

No adapter

[wookie1965]

39 minutes ago, moosestang said:

I've also tried using the 15mm with a 2x barlow.  This will focus ok, but almost all the way to the bottom of the focuser travel.  So there is a trend: higher magnification requires the focuser to be lower.

No adapter

Very strange have you got a local astronomical society you could take the scope up to or try a different length e.g. 8mm see if that focuses 

[moosestang]

I may have found the problem.  I discovered that the 6mm should have lenses in the chrome barrel at the bottom of the eyepiece.  Mine doesn't.  But the 15mm does.  I suspect the previous owner swapped the chrome part of the lenses.  Swapped them back, and now both focus at about the same spot.