| there are a ton of threads similar to this, so please do a search, but...
aside from rigid fingers whilst popping, you want your hand like rubber- very loose. it sounds like your popping technique is fine. thumping (like a ton of stuff) is 99% about timing. hit the string with your thumb and immediately pull it away. try this on an open string. i often play dead strings, so thats not your problem.
if that didnt help, search here, and search youtube. good luck. | 
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