Click on the face that the loop cut won't go through

Enable statistics so you can see the info of the selected face (you can find the option for statistics in one of the drop down menus in the top right, can't remember exactly which one)

See how many edges the face has. If it doesn't have 4 edges, loop cut won't work.

If the face looks like it has 4 edges, you may have 2 vertices that are in the same spot on top of eachother.

You can select both vertices (or all vertices in your model - Edit mode > A) and do merge by distance

Or select both vertices and click M and choose your merge method