i gave up on tablets and powder after i switched to 30 degrees, they always left some residue. i only ever use liquid wash now, in a little ball inside the machine and never have any problems. i use a scoop of oxywhite in with a 30 degree whites wash and this dissolves no problem.
i find the 30 minute 30 degree programme great and haven't used a higher temp since .....well, i can't actually remember when and i only ever use either asda's own brand or aldi's own brand liquid clothes wash.