Sure thing, Ramsus.
If you are using a reach weapon and there is one square between you and your target you do not slide it at all. When forced movement has a specified destination (in this case the square you left) and the target can not reach this destination (in this case beacuse the power specifies that it only slides 1 square) then the forced movement is negated (PHB2, p219).
This could be solved by using a staggering weapon or wearing rushing cleats, both of which increase the sliding distance so that the destination square can be reached.
Some other relevant points:
•With Overwhelming Strike you can decide not to shift, but then you can not slide the target either
•With Overwhelming Strike you can decide only to shift and not slide the target
•With reach you should be careful as an avenger because your oath does not work when not adjacent
•With reach you should be careful as an avenger because without the feat Barreling Charge you can not end your charge adjacent
In which case, you may want to get another weapon soon