As a windows user my .snippets-files are located at $HOME\vimfiles\Ultisnips.
I created some snippets to use in .txt-files and stored them in a file named txt.snippets.
When i created a new .txt-file they weren't available. So for testing if they worked at all I renamed the file to all.snippets, as this file didn't exist before, and they suddenly worked as intended.
Is there another way to make .txt-snippets work besides adding them to all.snippets?
The foo in foo.snippets doesn't refer to the .foo extension, but to the foo filetype, that Vim may or may not derive from the .foo extension.
In this case, Vim assigns the text filetype to files with the .txt extension so your snippets, which depend on the txt filetype, are not active.
Renaming your snippet file to text.snippets should solve your problem.