It sounds like your blade is dull. Try a new blade. Your jig saw blade lost its set. Or if you are using a circular saw or table saw - same thing, you need to either sharpen it or buy a new blade.
In this project, I believe a table saw is the way to go. After you cut the planks to length, that is. Then, I would use a table saw to rip them to the correct width and put the grooves in them. You can get the dimensions for the length, width, and side groove sides from GMC Pauls website. This is very helpful information.
Good luck.
