How Do I fix a wall outlet that has low power (but does have power)?

Since other outlets work on that circuit it is not the breaker. Those cheap testers are very unreliable and don't necessarily give you accurate results. I suspect there is no voltage to that outlet. Is it on a switch? Is it last in the chain? If so you need to check the working outlets to see if one of them has a bad connection heading to this outlet.

Call a qualified professional electrician to locate and repair the open neutral on that unit. I suspect that you have a circuit fed from a double cable with a shared neutral and you are back feeding the dead outlet through another device. Not an easy item to locate even for a top trouble shooter.

If you have changes the outlet and still have nothing, your next choice would be to change the breaker that it is wired to. If that don't work then their is a problem with the wiring itself.

Switch off the mains at the switch box. Remove the wall socket. Check for loose wiring.