Most common fault with stop/tail bulbs and brightness issues, is the bulb being in the wrong way around. Rotate it 180 degrees. There are 2 separate contacts on the base of the bulb, one for the brake light filament (21w), the other for the tail light filament (5w).
If that doesn’t resolve it, you’re most likely looking at a bad earth on that cluster of lights. The brake (21 watt) obviously needs a better connection as the wattage and ampage it uses are greater.
If you can’t suss it, you’re welcome to pop over to me and I’ll give it the once over!