I'm using a veritas small plow plane and I'm finding my grooves are a bit trapezoid shaped and not square to the reference face and edge:
Material is Sapele. This is a longer piece (~48"). Some parts of the groove are better than others. The ends seem particularly bad.
I suspect part of the issue may be taking too heavy a cut with the 1/4 cutter. On the Veritas plow the 'wavy washer' doesn't seem to make contact with the outside of the blade to hold it against the bed, it seems to rely solely on the lever for that with the smaller cutters. This seems to cause the cutter to cant a bit and go sideways.
I'm also going to try an auxiliary fence.
What else can I try to aid in my diagnosis?