Some of the crock pot/slow cookers only have a low/medium/high setting on the thermostat. I recently called the company to ask what temperatures they correspond to. 200/250/300 degrees F was the answer. This isn't hot enough for hot dip.
Four years ago I purchased a Presto (~4 qt.). The thermostat goes up to 400F. I use it without the porcelain liner. If it is half full, it heats up in a few hours. If it's nearly full, it takes several hours. I usually have a batch of stuff to dip every couple weeks and I just let it heat up all day on dip day.
I purchased some mineral wool from McMaster and wrapped the OD of the crock pot, keep the bottom open. This helps reduce heat loss and reduces heating time. I believe that, given a combination of a full pot of dip, no blanket, and a cold shop, the crock pot probably doesn't have enough wattage to ever get up to temperature.