I think that the narrow pulse needs to be 1.5 milliseconds long or less, your 1.8 milliseconds may be to long a pulse for the low speed end. What is the repetition rate of the pulses? They should be about 20 to 25 milliseconds apart. The typical pulse size is 1.5 milliseconds plus or minus 0.5 milliseconds every 20 to 25 milliseconds.