If you do not manage to solve the problem after your next visit could you try these please (some of which have already been mentioned by others but included here as a neat summary):
a. Replace one of CFLs in the problem bollards with an incandescent lamps.
b. Replace both CFLs in the problem bollards with two incandescent lamps.
c. Try connecting only the problem bollards to the contactor if this is possible.
d. If the bollards are double pole switched by the contactor try single pole switching.
e. Use a temporary switch instead of the contactor to turn bollards on and off.
f. Use a switch instead of the photocell to turn the contactor on/off.
g. In the as wired set up connect an incandescent lamp across the contactor coil.
h. Measure the earth leakage current for all bollards, 4 bollards and if you can only the problem bollards.
i. Check the earthing of the bollards and the swa glands and nuts.
j. Do some ramp tests on the rcbo.
k. Check tightness of rcbo terminals. Is the functional earth connected?
l. Could you provide a sketch of the wiring including the contactor contacts and tells us the type of photocell and contactor.
m. Try fitting new CFLs to the problem bollards.
n. Inspect all connections for signs of tracking after the water ingress.
o. Temporarily swap over L and N to the bollards and see what happens when you turn on and off, first on the supply side then return to normal and then on the load side and then return to normal.
p. If there is another 6A RCBO temporarily use this to provide the supply.
q. Disconnect the L to the bollards on the load side of the contactor and switch contactor on/off, reconnect L and then do the same for the N to the bollards.
There is a better order to these but I leave you to sort that out.