A solution is to get rid of the knots and to make splices instead. Splices do look cool and don't catch on anything.
I'm assuming your jib sheet has a 2:1 purchase? In that case not only the knots can catch up, but the blocks can do the same. You can make the issue less problematic by playing with different length of pigtails.
Also I noticed when sailing with my son that the jib has a tendency to snag when he's sheeting it too early. You know kids, always impatient: On a tack he release the sheet, jumps on the other side and pull the new sheet hard right away. And promptly snag it on the mast. I've shown him that waiting until the jib passes over before sheeting in is a better way to go
So you may want to review your tack timing, a little change on when you release or pull the jib sheet can prevent snagging.
Finally you mention a spi line, so I'm assuming you have a spi pole as well. You can try to tie a bit of shock cord from the pole to the mast so the blocks go around the shock cord instead of around the mast.