Please HELP I'm lost!

I have two motors controlling 2 extract fans from a panel housing the two contactors and relays sat next to the trend bms interface. Contactors are 3phase with overloads and auxillary's with a 24v control, contactors fused via 3 separate cartridge fuses

Fault occours on no.1 fan, found yellow phase fuse blown and contactor fused so changed both, tested overload no problem, tested motor, wiring relays and control voltage all fine!
no. 1 contactor will not stay in on auto or hand, I reset via bms, the contactor comes in then 2 seconds later drops out on fault??

I've run out of ideas,

To add to that, I've already increased the setting of the overload from 3amps to the max which is 4amps, the flow sensor on the extract fan is fine, there is no difference in voltage between phases or to earth, all connections are tight. The extract fans are in one unit so the damper will only allow 1 to run at a time and is opening and closing fine. Mechanicaly the motor is fine because I've run them both off the no.2 contactor for some time and no problems, what could possibly be make the contactor come out?? The insulation resistance test on the wiring and the motor was as high as could be

Please please please someone help I want to understand this problem

Try replacing contactor #1, since both motors seem to run fine off C#2.