You really are going to have spell out exactly how you are making your emulsion. The ingredients are only part of it. A useful analysis also needs time and temperature at each step. They all have an effect. The good news is that you don't have "peppering" (black precipitate}. That is usually the bugaboo. We should be able to get you a full max black. If you're around today, I'll try to watch APUG so you can get back to things asap. If you can supply more information, I'm sure other emulsion makers will weigh in also.
One thing: this thread has managed to confuse you about contrast. Contrast is usually meant to mean the number of discernable density steps from black to white, i.e. how many shades of gray. Addition time does influence that. In general, the faster you add the silver nitrate to the salted gelatin, the fewer shades of gray between a solid black on one end and no-to-very low density on the other (with the right exposure and the right developer.) A slow addition will give you a nice, long gradient scale -- usually something that is preferred -- but it still should have the capability to deliver solid blacks. How much time and at what temperature you let the emulsion sit before you coat has a huge influence.
The stainless steel thermometer is not at fault.