I've had these minor leaks with Clark tarps:
Before reading, understand that I think they are as good as any tarp offered, and in some respects better designed and executed.
First. the nylon-plastic swivel sewn into the ridgeline may need seam sealing. But, you probably don't have one.
Second, the hem on one, at least, fills with water. I squeegee it out between a pair of fingers in 30 seconds. Clark hems are generous in size. I could seam seal a whole lot of feet of stitching, but then, it may just be harder to squeegee the water out, and that water is no problem
Your problem: It may be that running the ridgeline underneath is causing it.
Per other threads, mix your own silicone sealant, use seam seal for sil nylon, or find the Permatex product to seal windshields. It flows very well.
And come back and tell us about the rest of your experience with the NX-250.