There is a raised (dimpled outwards) ring around the cover tube to prevent the rubber seal from migrating outwards. If the outer clamp is not positioned on the crankcase side of this ring it will migrate. Try warming the rubber with a hair dryer, loosen both ring clamps and push the rubber very hard towards the crankcase then reapply the clamps. Also the cover tube may be crimped in this area or the raised ring may be worn down or manufactured too small.
Unfortunately you must remove the cylinder to replace the tube if it is defective.