Transition between uneven floor; reducer is wobbly
It just doesn't fit! I would suggest you measure every dimension of that transition very carefully, and make a rough profile drawing (pretty much exactly like your picture) to record what you measure. Take your drawing to some of the big box stores and possibly some specialty flooring places too, and see if you can match anything up (a caliper might be helpful). Those transitions are really tricky to deal with, so you might need to get creative. 

You could try something like this:


this will span the gap and handle the reduction, but you'll need some sort of shim to support it. For that, it might be possible to get a wood strip that fits in the gap area, or you could look for some sort of rigid, dense foam that you can cut with a knife.

The best solution is to find something that you like and that is close to fitting (what you have pictured would work perfectly) and cut it to size using either a table saw or a routing table. Of course, you'll either need to own the tool or have access to one somewhere. Both will require some feather boards and guides to get a clean and safe cut, but this way you are guaranteed a good fit.