It's a poor design, almost designed to fail so you have to buy a new assembly.

The circuit boards on the tail lamp assemblies are basically an exposed contact board that is held on by plastic tabs that are melted and smashed down onto the circuit strip to hold it down.

Those circuit strips often corrode and warp causing poor contact.

You can scrape the corrosion off where the bulb makes poor contact, but the problem will come back.

Also scrape and bend the tabs on the bulb socket itself to make better contact, but again, it will act up again down the road.

The best fix is to replace the assembly itself, but they have not been upgraded much and will most likely give problems down the road as well.