Using tinder fungus instead of charclothe does take a bit of adjusting in your fire starting technique.
Some people have a container with little chunks of the fungus in it, and strike sparks down into them. When one chatches, they then pick that chunk out to use starting their fire.
I take a small piece around the size of a peanut, and hold/pinch it with my thumb on top of my flint - kind of like I do with my charclothe. I then strike my sparks trying to get them to land on the tinder fungus. It does take longer for a spark to catch. From my experience, anywhere from twice as long on up to 8 or 10 times as long. So it does take a little longer to catch that initial spark.
And once you have caught that spark, you also need to adjust how you use it in your bird's nest tinder bundle. The "heat" it generates does last longer than in a piece of charclothe, With charclothe, the spark quickly spreads throughout the whole piece (as you gently blow on it). This greatly increases the "heat" involved, and available to transfer over into your tinder bundle to get it hot enough to start burning. With the tinder fungus, your "heat" is far more concentrated, but longer lasting. So transferring that "heat" to the rest of your tinder bundle can be not as concentrated. So you can "burn out" the tinder right next to it without getting the rest hot enough to catch fire. I often put an extra chunk of regular fungus (or even some charred wood) in my tinder bundle right next to the spark I caught in the tinder fungus. This then picks up that spark and extends it and increases the total "heat" as it also starts to glow. This all helps then generate enough total heat to get your bird's nest burning.
A good way to practice this would be to build your bird's nest, and then fish out a small ember from your campfire. Put that into your bird's nest, and then try to start it burning. That way you see the difference in using a smaller level of "heat" over a longer time frame as compared to using charclothe. It is different, and you need to adjust your technique, and also how you might build your bird's nest tinder bundle.
And it all takes practice. Practice catching that initial spark, and practice converting it into flame in your bird's nest tinder bundle.
I hope these humble thoughts help.
Mikey - yee ol' grumpy German blacksmith out in the Hinterlands
p.s. Who tried around a dozen times to catch that first spark in tinder fungus before I worked out a method that would work for me. Frustrating, but I worked through it. Now it is far easier. The big point is that tinder fungus (innonotus obliquus) will catch that spark as-is, without any prior preparation, and even fresh off of the tree. In my experience, the fresh tends to catch that spark a little better/faster than the stuff that has dried a bunch.