When I had this problem there happened to be a thread on here where someone suggested that, to check for crossover, remove scope, leave bottom clamps on and place a knitting needle in bottom of curve to check alignment with the barrel. I used a welding rod - a far more manly solution - it's a crude method, but I saw that my scope was way out.

Turned one of the clamps round but it didn't work.

Bought some BKL centreline mounts (available BAR I think). Should have sorted the problem... but...

...and there's always a but with such problems...

...I discovered that, when I adjusted the parallax on my scope (an Airmax), the POI would change. Sent scope back; got another; not much improved. Resorted to standard scope.

Now did the BKL sort my problem out, or was it the changing of the scope? I honestly cannot say, but now that I've got it sorted I ain't changing anything.

To muddy the waters still further (sorry), with a non-parallax adjustable scope, failure to seat your cheek on the stock in exactly the same position for 55yds as for 35yds will cause an offset aim. But I think the latter problem may cause a more scattered grouping (or to be more precise, a couple of mini groupings within a larger group) rather than a neat offset.

Sorry for the rambling, it's just the mood I'm in tonight. Hope at least one piece of some of what I've offered may help.