I have the rudder lift kit which is quite handy for wet sailing. But I did run into the same issue. I got this suggestion from Harry at Flying Scot:
It sounds like the slot has tightened up for some reason. If you have tried lubrication with wax etc. and it is still tight, the only fix is to sand the slot a bit wider. I have done this with a piece of 1/4″ plywood cut to a 36″ X 4″ rectangle. I glue a piece of coarse grit paper to each side to make what amounts to a really big file. With the blade out and the head in a vise, I sand on the slot until the blade fits well and moves freely.
Also, I have had to replace the shock cord on the lift kit.