Anyone know anything about sandboards and sandboarding in general?

[Bushbasher]

My kids wanted sandboards for christmas but as I couldn't afford $300 each for boards I decided to have a go at making some. I started with a couple of molded plastic snowboards and glued some thin ply to them then glued formica to the ply. They worked a treat but had no directional stability so I made a couple of tail fins for each board, not too big, just deep enough to stop the board slewing sideways once moving. They worked ok so it was so far-so good.

Yesterday we went out into the national park to test them out and took some candles to wax the boards, as well as some proper sandboard wax and also some silicone spray that happened to be in the van. From the first run there was a speed difference between both boards, a yellow one and a purple one. The yellow one started out faster but as the day went on and as we tried the various waxes and spray, the purple board got faster and the yellow one got to the point where it just wouldn't slide at all

I'm at a total loss as to why the yellow board stopped sliding as both are basicall identical and had the same waxes used on them at different times. I thought there might be a residue issue with the yellow board so polished it back to just the formica finish but it still wouldn't go so now I'm asking if anyone might know something that will give me a clue.

Also, I noticed that no matter what was used, the boards would need the wax re-applied after every run, is that normal? Also the candle wax was faster than the shop bought board wax, is that supposed to be right ?

If anyone can shed some light on my delema I would be most gratefull.

Cheers
Bushbasher