Well, it definitely isn't the fact that propane is your fuel.
Narrowed down, here are but a few things to consider.
1. Your chamber construction. You want insulating qualities. Different materials will have an impact on how much heat you can sustain. Hard firebrick, although more durable, will soak up more heat than the soft insulating firebrick. Generally, the ceramic blanket has the best insulating properties.
The reflective coatings like ITC 100, Plistix, ect. really help out a lot, too.
Bottom line here is that the more heat the walls absorb means less heat to your work.
2. Sizing the burner to the chamber.
I've only heard of and seen photos of the 2-brick forge; I've never built one or seen it in action.
But a given is that the burner must possess enough BTU capability for the chamber size and work in question.
Here's a crude example: Burn a candle in an open kitchen oven, then burn it in a tin food can.
Or another example: Watch a plumber solder a pipe joint. The two considerations are that a.) heat rises and b.) air is an excellent insulator. While soldering the joint, one can have their hand relatively close to the work. But the pipe itself is very hot because the flame is directly on it.
With a forge, you are not training the flame directly on a small work area; you are attempting to heat the contents of the chamber, and that includes the air in it.
3. A very real possibility is attempting to exceed design limitations of the forge. In other words, asking the forge to do more work than it can do.
I have a "bean can forge" I use to make small tools like chisels and punches. Many of these are made from small stock like 1/4" drill rod, etc. But I found out very quickly that sticking larger stock in there, even though it would easily fit, would not get up to forging heat.
4. Yet another consideration is that maybe you're relatively new to forging (like me), and you may be getting the work hot enough, but just don't have the experience to know the difference.
If you can give an indication of the color the steel is achieving, it may help narrow it down.
The MAPP fuel is good for applications requiring an intense, direct heat. Same thing with fuels like acetylene. Not so ideal for applications where heating a volume is a consideration, mainly due to cost of the fuel vs. the heat obtained.
MAPP may make your forge run hotter, but first consider the size of the forge in relation to the size of your work. (You haven't mentioned size of your stock.)
Larger work needs a larger forge, and a larger forge needs more fuel. At some point, more fuel means a larger burner.
Good Luck,
Rob