Hello Gill
Was the piece of wood you had problems with roughly straight and flat to start with?
If it is cutting the leading edge but not the trailing edge, that sounds to me like the wood must be a little bowed and you have the convex side downwards.
As you move it over the planer, you might be rocking it forwards (as you'd naturally do since most of the pressure is near the cutterblock). This will lift the rear part of the timber so that it misses the cutters.
Thing is, with each pass, the planer should be working its way through the bow, so that it is nice and flat after a few passes... :?
As a general rule, plane the concave surface first, then go for parallel at the thicknesser. (Granny, eggs, suck... :wink: )