Powder left in drawer after wash - usually. Tried a few things

Hello,

I have a Hotpoint washer dryer.

USUALLY there is powder left in the drawer at the end of the cycle, a wet gunky mess. Occasionally all of the powder will disappear but I can't see a pattern.

I run a maintenance cycle every other week.

I do not use fabric softener, just white vinegar. I follow dosing guidelines precisely. We are in a hard water area.

I cleaned the soap drawer , and pulled it out and cleaned the bit inside the washing machine too (fine powder build up here too) , and I also cleaned each nozzle just in case.

Still leaving powder!

I'm not sure what to try next. My machine is built into the kitchen, it came with our 2 year old house. How do I go about getting behind a built in washer dryer? It is located underneath the sink draining board. I tried to clean a filter at the front once but it's behind the kitchen kickboard thingy which is siliconed to the floor!

Many thanks if anyone can help! My family has just had a sickness bug so I really need to make sure everything gets cleaned properly.