after using nela s*p*r*lex trowel since they came out recently I've noticed on the butt end of boards they show lines where the boards join onto the other boards .my method of using the trowel is 2 cross trowels at the end of the gauge but I do lay on only 2 mil !my question is is it because I'm laying on to thin ? Or just maybe hitting to soon with flex any ideas very much appreciated thanks in advance .