If the system is low on refrigerant, the indoor unit will not be capable of producing the right amount of heat. Therefore, it will not allow the air handler to ramp up the fan speed.
A quick test to perform to verify if your system is low on refrigerant is to set the unit to the maximum heating temperature of 86° F (30° C), run it for 15 minutes and check the larger pipe coming out of the outdoor unit. It has to be at around 105° F (41° C) which should be very hot to the touch. If the pipe is not very hot, then your system is low on refrigerant and you could possibly have a leak.
To know how much refrigerant your system needs, you can refer to this link.