Weird problem when using relays
The relays you're using aren't sealed, and over time - depending on the environment in which they're being stored/used -  their contacts can become oxidized or contaminated with chlorine or sulfur, causing  their closed resistance to become higher than specified on the data sheet.

If light, the corrosion can usually be blasted away and the native silver alloy exposed by hot-switching the contacts at their maximum rated voltage and current. If that doesn't work, then the contacts must be mechanically cleaned.

The reason these relays are failing for you is because their contacts are made from a silver alloy especially formulated to switch highish power, and the low power levels you're using don't have enough "oomph" to blast off the contact contamination and get down to the bare metal.  For reliable operation and maximum life of the relay you should switch to a relay designed to handle signal-level power, and it should be hermetically sealed.

For your perusal: