I have also been trying the tautline hitch and it still didn't have the results I was hoping for. I tried the additional turn in the tautline and it held better but still would still eventually slip as the tarp tension changed due to the gusting wind on the tarp.
After playing with it for a while I did notice that it started to hold a bit better where the fibers had fuzzed up a little from previous friction.
I did read on wikipedia the following on the tautline hitch which is inline with what Caboyer mentioned.
These hitches should not be depended on to hold fast under all conditions. With some particularly stiff or slick modern fibers (especially Polypropylene commonly called 'poly' based lines) they can be difficult to make hold at all. Sometimes it can be made more secure by using additional initial wraps and finishing half-hitches.
Caboyer, thank you for the weight analysis test. It sounds like the cord would be more than adequate.
Warbonnetguy, it looks and feels like nylon but the packaging definitely is a little ambiguous around the composition. So it could be a mix. I will go back to HD to see if I can find one that says it is nylon.
I will have to try the additional wraps and a finishing half-hitch to see if it helps. If not it looks like it may be time to finally move to one of the other systems you fine gents have recommended.
Cannibal any websites or tutorials on the MSR Wrap?
Check these links out, some of the knots and hitches may be helpful. I have used small diameter slick nylon line in these applications and have had good success.