As Lee said, it may well be a temperature problem, however, there are other things you could try if you are confident your thermometer is OK:

1. Bracket your exposures very generously on a test roll or two and see how things fare on those at + 1/2, +1 stop, +1.5 etc or so. You may well know, but it can be advantageous to slightly overexpose (same as rating your film slower ie an ISO 100 film at ISO 64 or so) to ensure you have good shadow detail - many people do this, often accompanied by a slight reduction in development compared to the manufacturers time (-15% or so). As you have flat negs, however, try overexposing but do not reduce development. If you bracket when using slide film, which do you generally prefer, those at no compensation or those at + 1/3 or + 1/2? If you are choosing 'overexposed' ones them your camera may have tendecy towards slight underexposure. This might give more trouble with B&W where shadow detail is key. This compounded with rating film at manufacturers speed could, possibly, be leaving you a good stop or so underexposed. If you find, for example, that your negs are best at +1 stop, rate that film in future at ISO 50, instead of 100, or 200 instead of 400 etc.

3. You could have a water quality problem, but this is unlikely to cause big probs with regular developers.

4.Developer fresh and diluted correctly?

5. If none of this helps to put acceptable contrast into your negs, try boldly increasing your development time (+25% to start with) and fine tune from there. Without wanting to generalise too much, if you have good shadow detail, but flat highlights, you are underdeveloping but correctly exposing (or reasonably close). If you have poor shadow detail, you are underexposing.

Experiment until it works for you. Do not assume that if you develop for 10 mins and the manufacturer says 8, that it is wrong. If it prints right, it is right (for you). Differencess between your times, etc can occur for various reasons; one person may develop their film for far longer at apparrently the same ISO, same temp, same agitations that another person and get similar results!

Remember that more than one problem could be present ie you could be underexposing by a stop and underdeveloping by a fair %. This would result in poor to zero shadow detail and very flat highlights.

Also remember to change only one thing at a time to ensure you know which change is having the effect.

As a last resort, try sending film to a lab used to dealing with B&W and of good reputation and see what the results are like....

Let us know what the problem was when you find out!!!