It is just hard to get the heat from a burning/glowing chunk of fungus to transfer to dried grass. The individual stalks of grass just don't pack together as well as you would like. So you end up burning back a number of stems/blades to create a "hole" in your tinder bundle - before you get enough combined heat to turn into flame. It can be done, just is hard. With grass, you really need to pack them close together
Plus grass burns up faster than shredded bark. The bark is just denser, and will hold that heat/spark longer - especially when gently blowing on it. That's why I prefer shredded inner bark from cedar trees.
The rest is just a matter of practice and experience. Some people have few problems with dry grass, while others never have much luck. It just varies depending upon your overall technique.
Here's an historical note about tinder for fire starting.
This quote is from a section talking about a shortage of rags for paper making in England.
Quote:
The Monthly Magazine or British Register. Vol. IX, Part 1 for 1800, from January to July, inclusive.
Instead of making tinder from rags, it might be made, as in France, from the agaric of the ash. The fungus is diced thin, dried, beaten with a hammer until it becomes very elastic, and boiled in water impregnated with salt-petre. in this state it readily catches fire from the spark of a flint, and is sold very cheap at the shops by the name amadou. If this substance were ealily to be had, the rags now burnt for tinder might be preserved.
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There was some debate about this being a reference for making and using charcloth, but my opinion is that they were using wadded up rags for their "bird's nest" tinder bundle - especially in the cities and towns. Access to dried grass/leaves or even old rope to make a "bird's nest" tinder bundle would have been in very short supply in areas with a lot of people needing to use it to start their cooking fires. So my opinion is that they were using their rags to make their tinder bundles for fire starting. Making charcloth only takes a small amount, but a tinder bundle takes many times more of it. And this was when they still made paper from rags - well before they started commercially making paper from wood pulp. So the "shortage" got bad enough, and the need for paper had increased enough, that people were concerned about not having enough rags available to be bought up and made into paper.
Just a little historical ... sidetrack ... along this fire making journey.
Mikey - that grumpy ol' German blacksmith out in the Hinterlands
