Migration of meshes doesn't work properly

I’m new to unreal, and I’m trying to get some assets from another project I downloaded (a vegetation pack). However, when trying to migrate the meshes needed (like grass, bushes, flowers, etc.) from that project to the one I’m actually working on, it looks like their materials are not migrated properly.
I am sure I did the migration properly, as instructed on the documentation page. I selected the assets that I wanted to move (in my case, the meshes), selected the migrate option from the right click menu and selected the Content folder of my main project. The migration completed successfully (according to unreal’s message log, at least), but when checking the contents of my project, the meshes were lacking their needed material. The default world grid material was selected instead, and although the materials those meshes used at first were too imported, they were pretty much just empty, with no textures or anything linked to them, just the way a new material would look when opening it.
Do you guys have any clue why this is happening? Thanks!