I don't often fly the C172 but thought I would take it for a spin after the latest MSFS updates.

I have the Cockpit Interaction System set to Legacy as I mainly use the mouse to set switches etc.  All seems to work fine except the Trim Wheel and the Fuel selector won't move.  I asked my brother to check his C172 and it works normally.

I have deleted and reinstalled the C172 but that made no difference.  I don't have this problem with any other aircraft.

Any ideas ?

Since SU5, the C172 trim is very odd.  It makes a clicking sound when you move it (which is not true to real life). Also it is stepped rather than smooth, so not always possible to trim for the exact speed you want.  Before SU5, it was perfect, natural and lifelike.

I agree with OP, the trim in the 172 is totally broken and currently useless.

we will have to wait for a fix as it’s one of my favourite aircraft 

I own the Saitek TrimWheel, too. Did you try deleting all assignments (including Selectivity) and reassign them? I did this and it's working properly in the C172 (and C152) again. 

Besides I have to dial back sensitivity in both directions heavily, otherwise it's much too sensitive.

Another hint: Check in the Device settings if it's properly working in advance of entering the Simulator. It doesn't from time to time on my system, why ever, and that's an effect I've been observing for ages back to Prepar3d times and prior systems.

Kind regards, Michael