I ran into a similar problem where during the installation, someone twisted together the solid copper wires. These later failed with a high resistance connection. I have many years of experience with wiring (former electrical engineer), and the best way to connect SMALL SOLID copper wires are soldering, or crimped connections. Wire nuts typically are not designed for the #22 wire used in these systems.
PS the problem at my house was under warranty, and caused several trips for the repairman before the splice was found hidden under the insulation. Warranty, had new wiring installed to fix this.
I hope the above information helps.
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Pennsylvania, for 60+ years, most recently, Allentown, now TV. 
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