About a year ago I logged a problem with the key assignment of prop levels on the Dash 8. You can assign the prop controls to a keyboard key or joystick button and both work with other FS2 prop aircraft like the Baron or King Air but not with the Dash 8. This is an introduced problem as it initially did work on the Dash 8 when it was first released, but was later disabled by an update I assume. When will this be fixed or is there a work around I can use?
I'll take a look why that is not working anymore. The q400 uses discrete levers whilst the others use an "analog" ones.
Do you have any other controls assigned to the propeller speed? E.g. do you have an axis assigned as well or just the buttons?
Thanks. I initially assigned the prop +- to 2 adjoining buttons on my joystick. Then whe that didnt work also tried assigning to 2 keyboard buttons. I used  as they didn't appear to be assigned to anything. The only axis I have is the throttle so didn't assign it to any lever.