Techie question

I have a Meade DSI set up with a DSI wide adapter and a Tamron lens. I have
taken several pleasing photo's with this using Meade's Autostar DSI program
ranging from short multiple exposures to long timed exposures both guided
and star trail effect.
My problem is that I can only find focus at one position on the lens and
would like to know why.
The lens and setup are as follows:
Tamron 35-70mm 1:3.5 64'-34' ( ' means degrees!!)
It will only focus at approximately the 55mm setting and focused to 0.4
meters.
This lens works perfectly at all ranges on my SLR.

Any ideas please.