I just got a MacCat tarp and decided to try mason's line to keep my total tarp weight down. I'm having problems with knots slipping. Bowlines at the corners have come undone. Tautline hitches won't hold at all and even clove hitches have slipped. Anybody have suggestions?

When you say "masons" line, are you talking about braid?
I've used all the knots you mentioned with good success on braid line, sometimes called 'contractors line'.
There are many fishing knots...blood knot, surgeon's knot, improved clinch, nail knot, trilene knot, etc. that work great.

Might give a 'truckers hitch' a try, first. Easy and not permanent.

On the corners, a bowline would probably come undone. Bowlines like a constant load and a springy piece of string will untie itself. I use two half hitches with a stopper knot on the end, for eyelets, or tie directly to the fabric corner with a sheet bend.

Try the Anchor knot on the corners.

Mason line is fiddly little stuff. Braided works better than the other for our purposes. I use thicker rope for guylines, but if I switched to mason's, I'd tie a loop every few inches and just use the appropriate loop to adjust length - if you bought a spool of it, you have plenty of line to play with.

Thanks for all the suggestions. No, it is not the braided kind, just twisted. I did eventually get all my knots to hold except a tautline and a truckers hitch. The clove hitches had to be under constant tention. As for bowlines, I tied those in the corners because I have never had one come undone before. Two half hitches will untie themselves if not under tension, but bowlines was a new experience for me.

Instead of the bowline, try a fishing knot, like Dryer said. Fishing knots are designed for thin, slick nylon line that is not under tension all the time.