Prosim737 Generic Serial driver and MCP

Hello all, using my arduino, I am able to turn the AT arm switch on and off, among other switches by sending a string over the serial port. I cannot however change the values of the heading, course, Altitude, VS, etc...
I assign a string such as MCPALTinc:, and MCPALTdec:, and by sending either one, the value does not change. I am assigning these values to MCP ALT VAL inc Pushed, and MCP ALT VAL dec Pushed respectively.
Sometimes the Alt will change, but only once if it is the very first thing I do after restarting Prosim737.
Am I wrong to assume that MCP ALT VAL inc and MCP ALT VAL dec are for something else entirely? It would be great if these could be made to work with the generic serial driver.

Update::
Marty helped me via IM, I need to also send an off command to complete the complete on off cycle.