i wash the washing machine with proprietary products every couple of months, or after lots of dark items which might leach.
there is always some residue in a machine, which is not visible.
i have read that it is better for the machine to use powder rather than liquid detergent, and i find that aldi or lidl products are very good.
also i often do 20c wash as it is shortest, and some of the gel pods are not advised for below 30c; i guess they don't melt so well.
and of course, remember that these are very attractive to young children, who tend to bite them, as they look like jelly sweets. lots of incidents to emergency dept, unfortunately.