Those relay modules are notorious for bad solder connections.

If you have any soldering skills you should be able to fix it yourself. You need to remove the silicone sealer that they use to weatherproof the module and then you can pry it apart.
Resolder all of the connections of the relays that you will find inside where they mount on the printed circuit board. You will likely be able to see cracks around the terminals.

If you aren't confident with soldering, any TV and radio repair shop will resolder it for you for a reasonable price (should only take a few minutes).

Those circuit boards are wave soldered initially and the process does not put enough solder on larger items such as relays. Thermal cycling and other stresses eventually cause the solder to crack giving intermittent conditions.