Trouble rooting my red ludwigia

I've heard of some people being successful to some degree, with substrate that large, but without doubt, plants root and grow better in a finer substrate than what you describe.
I would stay away from sand because it is too fine, but something a bit more coarse would offer your plants a better growing medium than what you're using.
How deep(in inch's) is your substrate? Maybe you can simply add some finer material to what you currently are using. The problem with that is that eventually all the fine stuff will sift down to the bottom, leaving you in the same position you're currently in.
BTW, reaching in with your hand and gently moving branchs around would be a better way to go than using a net.

Len

Some people use 'play sand'. The stuff that they put into sand boxes for the kids. I personally have no experience with that medium except to mix it with other forms of substrate.
My inclination is that it would be too fine by itself, but those with more experience with it will probably chime in on that subject.

Len

I used to have the same gravel as you and had lots of problems keeping plants down. I have since added some finer Flourite to it and it helped. I found that if you plant the plants and push the gravel around to compact it they will stay down a bit easier.

I've heard stories of people having good success with sand but have not personally tried it.

My experience ( very little ) is similar to yours, right now I have 3 tanks up and running, all with plants. The big tank has a meduim red flint substrate the other two have fvery fine gravel. I had a real devil of a time getting my crypts planted in the medium gravel and the fine stuff was very easy to plant in. I had constant floating crypt syndrome for a good week, and finally got things in so I hope they will stay. I'd try to find some fine stuff, or if you can match it mix some but as was said eventually all of the fine stuff will end up at the bottom.