Sounds like poor technique on the p/t to me. If you have one edge that is straight you could put it through the thicknesser on edge which should give you a straight edge on both sides. If not, use the planer to take a couple of light passes starting sbout one third the way along, stopping at two thirds with the first then another covering abot three quarters of the edge. I n doing this, try to keep even pressure avoiding putting pressure near the ends. Finally take a complete pass to take out any remaining curvature near the ends.
An alternative approach is to take out the convexity using a hand plane , starting with a short pass over the centre of the edge, then gradually taking longer passes fiinishing with one over the full length of the board. If this process has left the edge slightly out of square you can then square it up on the planer.
Having said all that, I do not consider a board to be ready for edge jointing straight from the planer. I always plane them on a shooting boaard aiming to leave the edge slightly concave over its length. The idea is that the ends of a board tend to dry out and shrink more than the centre and this slight gap in the middle (closed up by clamping) counteracts this and reduces the likelihood of the joints opening up at the ends over time - you can often see where this has happened on old pieces of furniture.
Jim
