One factor to bear in mind - and I don't know how it applies for the case of the original post - is that the reverse thrust schedule (or the N1/EPR schedule, it comes to the same thing) with airspeed may be playing a part.
Some engines allow the use of differing levels of max N1 in reverse mode according to airspeed, due to concerns about engine gas reingestion and/or aircraft direction control issues.
So it may be that the loss of effectiveness at lower speeds was caused by a FADEC scheduling you towards reverse idle as you slowed down.