Fibaro FGD-212 weird behaviour

Hi all. I have two Fibaro dimmers that are connected to GU10 halogen lightbulbs. I am using a 3-wire connection. 3 bulbs are connected on one dimmer, and 6 bulbs on the other. When I power the dimmers on, I need to try several times before the light comes on, and I can hear a “humming” sound from the dimmer even though the lights do not power on. After the lights have been turned on, they can dim up and down like they should with no problem. But if I turn the dimmers off again, the same behaviour appears again when I try to turn them on. Any suggestions as to what causes this?