As others have noted, the spring may not be able to overcome friction in the longer cable run. However, in my experience, I've never found a brake who's weren't strong enough if the cables were running freely (front or rear) so check the cable.

The other (rare, but not all that rare) possibility is that one of the canti posts is a hair short so that when the mounting bolt is tightened it binds the brake arm before bottoming out on the end of the post. The easiest way to confirm or eliminate this possibility is to loosen both mounting bolts. If that solves the problem, the cables are fine, but the problem is harder to fix.

If the posts are the removable/replaceable type, ask BD for a new pair (or if not maybe a new bike). Otherwise you need to thin the stack of the moving brake parts, which can sometimes be done by removing a washer or replacing it with a thinner one. Or add a thin spacer to extend the end of the post. I've done this successfully using O-rings, which get mashed down as I tighten the bolt, or with home made washers cut from paper and soaked in oil. Some mechanics solve the problem by using loctite to hold the bolt in place so they don't need to tighten it to the bottom, but I'm not a fan of that, especially for a rental bike.