As others have said, Zing-It and Lash-It are both coated ropes so they slide better which reduces their holding capability. Over time this coating can wear off and the knots will hold better, but it takes a bit of time. IMHO these cords are way overkill for tarp use.
With any kind of friction holding knot like a prussik or midshipman's/tautline, you might be able to add a few additional turns to help the situation, but no guarantees if you stretch your ridgeline super-tight.
The Farrimond Friction Hitch that Phantom Grappler describes is essentially a multi-turn prussik used like a midshipman's hitch. Regarding rope friction,
The Animated Knots site has this to say:
I assume you went around the tree and tied the knot at the tree side with either a continuous ridgeline. You may have better luck changing this by moving the tautline closer to the tarp. Prussiks and similar hitches hold better when the load is perfectly paralell to the standing part of the rope. If the tree you tie to is large diameter the vee that is formed can reduce the holding strength of a tautline hitch. You might try a split ridgeline where the rope is fixed to the tree (with a timber hitch for example) and runs straight to the eye or whatever on the tarp, then loops back and is tied near the tarp end (a trucker's hitch wouls also be effective here). Basically, this is how most Dutch bling works: Stingerz, Flyz, and Wasps all end up like this. The end result should have the load pulling parallel with the rope from the tree. A half-hitch backing knot might also help.
Me, I just use a set of Dutch Stingerz and don't worry about it anymore.
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