What was done during the 'cleaning' of the gun?
Was there any disassembly of the action or other parts done,,or just a straight forward clean the bores, wipe down etc.
Does the LH hammer re=cock if you first dry fire it and then open and close the gun as normal?

With the bbls off,,look down into the cavity on the action flats.
You should be able to see the cocking arm extensions of both hammers

Here's link to a Flues RH hammer
https://www.hogislandgunparts.com/Ithaca-Flues-Model-Hammer-Right-p/ia357-82.htm

You are looking to see if that extended arm with the right angle bend is visible.
If that extended arm and bend is gone,,then that has broken off and the hammer will not cock when you have the bbls and FE in place.
That extension is lifted up by the opening of the gun. A cocking bolt in the bbls lug slides under those extensions when the FE is put into place.

That part in the link shows a braze repair to that part. That repair was done because the end had broken of cracked.
So it's not impossible.
I've seen a coupe such broken hammer extension on Flues. Both were during the Cowboy shooting rage.
I suspect the John Wayne flip open technique may have had something to do with it, extra strain on those hammer extensions as the bbls violently drop down and open.

If the hammer is OK,,then a problem on the inside like: The sear is stuck in the released position for some reason,,the sear spring is not doing it's job,,a broken sear tip and/or the sear edge on the hammer is damaged.
Any of these conditions can leave the hammer un-cocked even though it is being brought back to the full cock condition by the cocking mechanism above.
The hammer rotates back on the pivot pin, but then just follows forward and down as the bbls are closed.
Sear springs in the Flues are generally not a problem. Very short small dia coil springs. Each set in the socket hole in the trigger plate and held there for assembly by a friction fit in the hole.