with the car off and the air cleaner removed, take a look at the linkage for the fmx kickdown its the same as the C6. first see if you have a return spring that is attached to the throttle plate to a hole in the kickdown, that makes sure that the kickdown returns to the fully up position.
next manual push on the kickdown rod and use a flashlight and try to see the transmission or get under the drivers side and you should be able to reach up to the kickdown, and try to move it up and down and make sure its moving freely all the way until it stops. make sure nothing is loose down there.
now go back up to the carb and push the butterflys open all the way and see if the kickdown actually moves, its possible your kickdown linkage is not adjusted correctly or could be missing. if it moves but you can see not enough there should be an adjustment a screw head that you turn to set the kickdown timing.
when the throttle is fully open the kick down should be 100% engaged. you can play with this a little and get it to open early just before full throttle this will set the moment on the pedal when the trans should drop a gear down.
now if you go through this and when you drive the car the kick down still doesn't drop a gear down,, you may have something bad going on with the transmission valve body.
maybe its sticking or some body screwed up a shift kit installation ,, though its an FMX so there really isn't any aftermarket support.
its possible the valve body is gummed up and in need of TLC.
the gear ratios between the C6 and FMX are basically the same its just that the FMX weights a lot more since its mostly cast steel compared to the C6 made out of aluminum.
but first double check that kick down. next i would check the modulator, when in doubt replace it,, just be aware, if you pull the modualtor the transmission will start to dump its fluid in your face, so catch it in a bucket ASAP and get that new modulator installed asap, WATCH out for the little rod that goes into the modualtor DO not loose it and make sure its seated correctly.. the best is you have a partner under the car,, you pull the modulator and he jams his finger in the hole before the entire transmission dumps its fluid, that will give you time to setup the pin in the modualor and the new seal (if its a push in or a screw in type) then you can get the new on in and replace 1-2 quarts that you lost.. plus you get to see the condition of the fluid.
worst case is the trans kick down refuses to work, and you have to dump the trans pan and look inside, best case you rebuild the valve body worst case mechanical damage and a transmission rebuild.