Switch to TT levers

I've converted an old road bike to singlespeed with a bullhorn bar, for no other reason than having something a bit different. I've fitted Cane Creek 200TT brake levers but the existing calipers (Miche 57mm road calipers) don't disengage so the lever is not returning. It's the same on both levers and the calipers worked fine with the previous Sora levers it had.

Any thoughts on what might be causing the sticking? I've recabled them twice, loosened the levers a bit and WD40'd the caliper springs, all to no avail so far. Is it a compatibility issue?

Ta