Got in my new WRX last week and turned the key, could hear what sounded like spinning and grinding but no turn over. Went and bought a new starter solenoid, and replaced it. Car started for the next day no problem, 6 or 7 times. Day after I started having the same original issue. Battery, and alternator tested and good, car won’t start with a jump, but I can bump start it. Any ideas? Flywheel possibly?
Are you sure the starter is aligned correctly? While most starters don't need to be shimmed, it happens every so often where they are not exactly aligned correctly with the flywheel ring gear and they won't start the engine. It could be just the act of removing and replacing the starter during the solenoid replacement got it into alignment, then after several uses, it went back to where you started from and now is giving you the same issue. When the starter gear engages the ring gear, the tendency is of the starter is to push away. If the starter isn't mounted as securely as it should be, it could be adjusting itself. You could possibly look at the starter while cranking to see if there is any movement. If it is moving, you'd need to brace it somehow to ensure no movement is present during starting.
The next thing to check is to ensure the ring gear is in good shape. It could be the ring gear has stopped at a place where there is a worn spot. The worn spot could be caused from the starter not engaging properly. If there is a worn spot, the starter may not grab it correctly. If there is only one worn spot and the engine was stopping in a different place when shutting it off, it might want to start just fine. If the starter is not engaging properly, you could try putting a socket wrench on the crankshaft hub bolt and turning it 45-90° then try to start it again. If it engages just fine, this would be a strong indication this is what's going on. Then, if you shut it off and it doesn't want to start again, pull the starter and inspect the ring gear for a worn spot. Only way to fix this issue is by replacing the ring gear, which most often means replacing the flywheel.