Rob, there's nowt wrong mate; you're just seeing natural settling and void filling thereby as the stuff is 'processed' by bacteria and fungi into smmaller bits (humus). I have two 'Dalek' plastic bins, about 4' high and 2' across each, and I can guarantee 12" to 15" settling once one is brim full and left for three months to work its magic.
Yes, turning the compost is a good idea - I speak as a Biologist of 30 years experience - but NOT once per day!! I turn the completed bin ONCE and then leave the Dalek to get on with it.
Worms, get Tiger worms, BUT DO USE THE SPERATOR AND SUSPEND THE COMPOST/BIN ABOVE SAME - if you want to have any worms left at the end of the day. Black wheely bins, suitably modified, are the canine's wedding tackle when it comes to worms.
I can recommend B&Q or any competent garden centre's 'accelerator' for compost - it is usually a Nitrogen or Phosphate augmentation with "enzymes" thrown in...yeah right...and WHAT organism are the "enzymes" in then? I smell an innoculation of bacterial or fungal spores to kick start things...
The only things that don't come out as dark brown, crumbly, non-smelling excellence are woody twigs and shredded branches. I personally simply chuck 'em back into the next load...they are usually starting to 'spalt' and simply need time for the mycelia (fungus muscle to you) to break 'em up. As stick/twig fragments, they act as structural wotsits inside the heap and allow voids to develop - which enhances drainage and aeration. This permits aerobic bacteria and discourages the nitrogen-nobbling anaerobic lot. Or, you could just burn them and add the "potash" (yup, this is where the name comes from) to the mix?
My strawberries and raspberries are 'normous and prolific this year 'cos I enriched my clay-based, urban, distressed, lino-thin, choclate dust (sorry, soil) with a nuclear attack amount of the first two years Dalek production. Taste? Magnificent!! I dusted under my apple tree too, just sprinkled a few bucket-fulls on top of the soil, starting approximately a yard out from the trunk, where I estimated the root ball started and the crop DEFINITELY improved a year later, as Mr Lumbricus terrestris and his mates incoroporated their free nosh into my laughable soil(?) structure.
I'm not an eco-anything, nor a fastidious gardiner per se, but my Countryman Grandfather's gardening habits made an impression on me and most of it is common sense anyway.
Sam
PS Grinding One? "the smell will turn you"...that AIN'T composting mate. That's fermentation...different ball game,...anaerobic bacteria.... Suspect you've got FAR too much grass in there and are making mock silage.
The soil you add is a good idea and is the bacterial/fungal initial innoculation. There is about the same population of these two in a teaspoonful of soil as there is people in greater Tokyo....
Sam