Two units - very different RSSI - why?

Just installed a system with 3 emonTx and an emonPi. All is functioning reasonably well with one issue: For three units I get RSSI of -56 for the furthest - about 10m away with two floors & walls in the way - and -45 and -69 for two units pretty much next to each other at about 5m distance with one floor & walls in the way.

The unit with the low RSSI is missing data noticeably (no surprise given the rssi) but I have no idea why the two co-located units have such a different RSSI as they are twins with stock antennas, etc. There are no obvious metal shielding elements around these units.

Antennas on all units are vertical.

Any ideas what I should look for in trying to track this down?

It is possible that you have a dead spot where reflections are causing the signal to cancel itself. Have you tried moving the units by a few centimetres?

Thanks for the suggestion - I tried that and it doesn’t seem to have any effect. I’m wondering whether I have a problem with the unit, but it is actually working, just not very well.

Have you swapped aerials?

Does the low signal strength move with the device or the location if you swap them?

big difference as mentioned what happens if you swap radio 1( the furthest) with radio two ( weakest) - i would think if it a radio/antenna issue radio 2 in position 1 will not connect any more… or even have weaker signal… if so look inside and see if flux/resin blob is causing a shorting issue with your antenna or even a little moisture in the antenna itself ( swap antenna) or more likely a cold solider joint where the sma connector is

SOLVED! The problem was that the sma centre conductor was apparently not soldered to the PCB. Reflowed the joint and now the two units read pretty much the same (good) RSSI

Thanks for all the suggestions folks!

Glyn is reporting the fault to the manufacturer - hopefully this was a one-off occurrence.