it looks looks a pain of a job to me. I've had to clean off the odd small area in the past when I'd missed patches due to not seeing the wet grout colour when washing off. You have to be patient and give the grout film remover (usually acid based to dissolve the cement content of the grout) time to act and then scrub with an appropriate abrasive pad to dislodge/clean off the residues. It will probably take more than one clean over each area to get it all off as you can't see what's left until it dries! I have read on other posts (older on this forum or possibly somewhere else) that where there is a thicker layer of polymer modified (flexi) grout to remove then you may also have to use something to dissolve the polymer so that the acid can get to the cement but I'd try without first and see how you go.