Hi Christine
Try doing what Kev said first.
I had a bit of trouble as well with my barlow a while back.
Worked fine on my 6" dob, but when I put it into my Saxon 10" dob, it rendered my 2 inch Celestron barlow useless. I was incredibly annoyed as it cost me $175.
Just would not come into focus.
Would not go the full way into the focusser- the stop for it was shorter than my 6" dob.
Kev, good man, came to the rescue and sent me a spare 2" barlow that he had and all is good now.
My now rendered useless barlow came in handy a while back- I lost a screw in the grass on the Barlow Kev sent me-
spent half the next day running a magnet across the grass and separating blade by blade by blade of grass - lucky I have a high fence- I must have looked a sad site
. But, no luck so I took the screw off the Celestron barlow
. Needless to say, I shake like a leaf whenever I change my eyepieces now.
A carpet square under the dob has it's place here and not just to keep your feet comfy and dry. Bunnings are actually selling the perfect sized squares of carpet for under $30 at the moment.
I hope you sort out your issues soon, Christine.