Time ro redo that connection. One thing to check, many valves with a threaded connection are sized to accept a pipe directly into the valve body (i.e., the valve body acts like it's own fitting). So, most plumbers tend to solder the pipe into the valve rather than rely on a threaded connection that might leak. A proper soldered connection just doesn't leak. Especially with a lot of the fittings being imported, the quality of the threads and the fittings just aren't what they used to be, and unless you are careful what you're doing, a leak can happen.