The missiles work just fine, provided they are on target, so my first thought is to check your TMA and enable range.
The detection range for the seeker head is about 10nm, so at 10-12 nm you might have some issues with the target being a bit too close. You'll want a minimum enable range and unless you have the missiles flying directly at the target out of the tube, a wide seeker setting at well. The practical minimum range is probably around 7nm.
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