My furnace blower will not run at full speed when the thermostst is on the auto setting but it will run at full speed when in the on setting what could be my problem

This is normal to run at low speed in "AUTO" on the fan switch of the thermostat in the heating mode, when in the A/C mode it will go to high speed, to make it run in high speed during heating , turn the fan switch to "ON", most furnaces have two speeds of operation, low on heat and high on cool. Good luck, Mike