Close range null point?

[Hair Bear]

Hi,
In the last 40 odd years I've been installing radios in almost everything and thought I'd seen it all but this it a new one on me.
I have an old Mustang 1000 fitted in a harvester, and 6 tractors mostly running T800 or T600, and all runnuing through large springer type antennas.
All installations check out ok with less than 2ohms to earth on the mounts and swr readings below 1.5. Static testing to a known base station at a 3 mile range give a result of S8 - 9+, R5. All good so far...
In operation, these machines aften travel within 10 - 15 feet of each other at which point the the multimode radios appear to go 'deaf'.
Any suggestions?