Usually overspray can be 2 things. Clogged head or static issues.
Make sure the head is clean and the capping station rubber ring/seal is cleaned.
You may have to do a few ink fills and waste ink, trying to break that clog free. I have had success in a few unorthodox ways on getting head to flow again. 1. Fat foam swap with lots of cleaning solvent and lightly press under the head nozzle. Do not rub! This could loosen any in the screen on the face of the head. 2. Need a larger, squirt type syringe, from the capping station (inside printer) follow the waste tube towards the pumps. Most mimaki's have a silver fitting connector that connects the capping station to the pumps. If you find that connector cut the tube about 1/2" from connector point. Attach the syringe over the tube end. Make sure the head is parked over the station and SLOWLY pull the syringe. This will suck cyan ink forced through the heads. Once you pull about like 10CC of ink, pull of syringe and reattach the tube to the connector barrel. Then do a test print with cyan only.
Another thought could be is fabric is very static prone. Do you have a cooper tinsle on the rear of the machine to prevent static? This helps too.
Just ideas...