I just looked your unit up in Amazon - is it a handheld one? I had one years ago and threw sit out - wasn't impressed.
Now I have on order with Q 3-1 steam mop - V35155 - shown last week. It's a total unit - floor and hand held uses.
I would think: because your Bissell doesn't hold much water - constantly filling the bucket and immediately using that water/steam .... the water never gets hot hot. You get steam but not useful hot steam.
For your oven cleaning project - buy some SPRAY 9 - it's the best product for cutting grease - garage floor - tar from exterior car - etc. Spray on and leave for several minutes - take wet paper towel and swish grease away. Repeat if needed. It's safe and non-toxic.
Stove racks - remove to sink - use the spray 9 and repeat above
Let us know if any of this works.............................