I'd say it's a dry seal in the caliper. Take the caliper off, take the pads out, pump the lever so the pistons move out, then spray a tiny bit of WD40 onto the pistons, then push them back with a screwdriver, wipe the caliper out really thoroughly and puts the pads back in, then refit the caliper. It should work, if it doesn't then I'd look into whether the lever blade is damaged. Or the other way round, the lever blades easier to check.