John, that little oxy-fuel (actual MAPP or Map/Pro which is pretty much just propylene or what I've recently seen on some big-box store shelves labelled as 'MAPP' but which is just propylene with a different label because "MAPP" was a trademark and not the 'legal' chemical requirement of the gas) torch set IIRC is rather picky about getting the oxy-fuel ratio just right. You have to constantly adjust the gas knobs as you add more of one gas (oxy or fuel) you have to add a little more of the other to keep everything still working in the flame.
I still have one of those, haven't used it in a long time though. It was a very small flame IIRC, but it did get hot enough that with oxy-MAPP (real MAPP) you could get steel up to 'kindling' temperature and then turn the fuel off and actually CUT steel with the oxygen jet only. Nowhere near as well or as thick as a 'real' oxy-fuel cutting torch can do, but it did do it. Very short run-time on the little ~1 ft3 oxy cylinder too, and expensive for the amount of oxygen you get in the little cylinder.
As mentioned, check the tip for damage or an obstruction.
But even though brazing is useful and handy and all, I'd probably MIG or TIG weld the muffler back together instead of brazing it. Once you put brass/bronze brazing filler on the steel, you have mostly eliminated EVER being able to weld the steel (without the major pain of removing ALL of the brass/bronze filler).
(I did use my little oxy-MAPP torch set to attempt to braze a cracked muffler for a car back in the day, because I was cheap and didn't have a welder and the replacement muffler cost much more than the little torch set. IIRC, I was able to apply the filler and fill the cracks but the braze repair did not hold up well and cracked again pretty quickly. I think I did end up replacing the muffler. Thinking some more, I think I also used the little torch to do a braze repair on a stainless steel measuring cup where the factory spot welds holding the handle to the cup failed and that repair has held up.)
If you want/need to try a braze repair on your little muffler, PRACTICE first on some scrap of similar size and thickness. IMNSHO, use the port-a-torch instead of the little torch though. It is MUCH more capable (yeah, I have one of those too

), but it is also much more capable of melting a hole right through the steel muffler. Hence the need to PRACTICE.