I don't recall anyone ever observing the B string having any weakness of output, but on the high E it's possible. The smaller the string, the weaker to sound for sure, but it's possible the magnet is closer to one side than the other. You might try rotating the pickup 180 degrees and see if that improves things. Depending on the string spacing, it may be too wide for the space available in the shell with a bar magnet and coil. Is it the same with all 3 pickups or more pronounced at the bridge position?