many
dimmer cards allow for adjustment of the min and maximum dimmer threshold - via potentiometers. These do become dirty and I have seen dimmers come on and stay on this way, removing the control signal doesn't clear the problem. Once you have checked the SCRs then this is the next likely issue. Sometimes adjusting them solves the problem. But first inspect the pot to make sure they are not cracked or broken. Heat and dirt play havoc over time with these pots.
Most but not all dimmers using SCRs and a min max pot can be adjusted this way. If you can get an Installation Manual do so and read it carefully. If you are not sure do not attempt this. Caution is always required because there are high voltages in dimmer racks. Always remove your watch and do not wear jewelry. THe following adjustments involve low voltage electtronics but you would not want to accidently catch a metal wristband on something at 120Vac you can loose a hand.
You need to put a load on the
channel typically 1kW is required, then command the channel to 15% and adjust the min pot so the light turns on to about 15% - have another channel on at this level for a comparison. Then set the channel to 90% and adjust the max pot to give approx 90% brightness. If this does not fix it then you need someone who knows electronics and has an oscilloscope. Good luck.