Airburst explained one possible cause, but there's another likely possibility, especially if an older more worn brake doesn't release even after a full stop.

The arm rotates an oval (sort of) cam between the shoes spreading them apart. Normally you never reach the tops of the oval, or the arm would rotate past that. When you release the lever, springs on the shoes pull them together rotating the cam back with a sort of wedging action.

But with shoe and/or cam wear, the cam rotates farther, getting very near the top. That means the the wedge angle is flatter, and with wear can be nearly flat, so there's not enough wedging action to push the cam back. The usual fix is new shoes and sometimes a new cam, but new shoes for the British made brakes are very hard to come by (at least here in the states, so you may be SOL.

If you're a fairly skilled mechanic, open it up, check for wear at the key points, and cross your fingers if anything is worn out.