George,
Let your doctor know about this. Anytime a prescribed medication isn't doing its job, attention and reevaluation is in order (especially because many drugs can have side effects that certainly aren't worth risking if the drug itself isn't even working how it should or was hoped to). Also see:
Nocturia: Is It Nighttime Incontinence?
http://www.healthcentral.com/incontinence/c/45/12997/nocturia/
Best of luck. - MG