KIM
The su should increase chuff rate as the power increases, so try disconecting the SU from the loco and just power it it from the digital set up , even a couple of cables taken from the track or track feed will suffice. make sure the digital system is set to the address of the SU and it should work correctly,
The problem could arise from the location of the power supply you are takng from the loco, if its an older loco , there may well be capacitors interfering with the supply, if poss try to locate where the pick up from track feeds into loco decoder.