In terms of testing the cabling there are two separate tests that need doing :
1) Check that (with all the heads removed) there is no connection between any of the four wires (including earth). If (say) the interconnect was shorted to the neutral then the interconnect wouldn't work and you'll get the symptoms described (only the one unit will sound).
2) Check that there's a complete link along each - i.e. L, N, and interconnect is connected between all the bases. As already said, if there's a break in the interconnect then the units will work in separate groups - those one side of the break, and separately the ones the other side of the break.
And for this size on installation, there really should be a test/isolate switch which for a hardwires system would be the one littlespark posted a photo of in post #76.
1) Check that (with all the heads removed) there is no connection between any of the four wires (including earth). If (say) the interconnect was shorted to the neutral then the interconnect wouldn't work and you'll get the symptoms described (only the one unit will sound).
2) Check that there's a complete link along each - i.e. L, N, and interconnect is connected between all the bases. As already said, if there's a break in the interconnect then the units will work in separate groups - those one side of the break, and separately the ones the other side of the break.
And for this size on installation, there really should be a test/isolate switch which for a hardwires system would be the one littlespark posted a photo of in post #76.