Have you try giving it more slack for the brake cable? There is a screw which holds the cable on the cantilever arm. After you unhook the brakes, loosen it, after you get your wheel on, adjust how much gap you want between brake pad and rim by holding it with one hand and tighten the screw back up with the other hand.
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In addition to the suggestions above, some cantilever brakes have two pair of concave/convex washers, a pair on either side of the brake pad mounting arm. One pair will be wider than the other. By switching the pairs, you vary the spacing from the pad to the rim. The concave/convex part is to enable orienting the pad to contact the rim properly.