Good coagulation comes from the denaturing and then precipitation of proteins during the boil and chilling phases, respectively. To get a good 'hot break,' as it's called, you need a vigorous boil and a quick chill. If the boil is weak, the proteins may not denature enough to fall out readily, and if the chill isn't strong enough they may just dissolve rather than dropping out. One of these things may be the problem.
Also, different grains have different protein levels, and different mashes form more or less proteins. You could just have batches that didn't have much to fall out in the first place. A protein rest during mashing may make a big difference, for instance. So will using wheat.
If the beer was clear in the end, I wouldn't suspect the lack of precipitation was a problem. That usually means the proteins fell out on their own with time. If there was a strong chill haze to the beer, accompanied by a strong head/mouthfeel, then you may have more proteins in the beer than desired. You could try using precipitating agents like Irish Moss if you aren't already, a protein rest during mashing if you're doing AG, and a stronger boil and faster chill if that's at all a problem.
