It sounds like you have the junction box version.
The junction boxes are designed to be waterproof, but make sure you use wire that is within the specified diameter.
Coating with silicon grease enhances the waterproof integrity and is worth doing on the rubber gasket
, the wires where they enter, and on the screws.
Finally consider the potential for water
to run along the wires a drip loop, or low point in the wire helps considerably.
Many people also coat the connections inside the junction box.
Finally get a clamp on multimeter so you can check the output of each panel separately without opening anything up. This will quickly show is there is a bad connection especially in a parallel configuration.