We have been using Tesco value soap powder for years. Strange how you mention powder though, as reacently the powder does not always get completley washed out of soap compartment. Water flow into the compartment appears to be ok.
It can take between 30 secs and 1 hr (most annoying) for the interlock to lock the door.
When it was working correctly the interlock locked the door within a few
seconds of the power button been pressed.
It sometimes takes ages for the interlock to lock the door when the machine is switched on. Once door is locked the machine runs ok and releases the door ok when finished.