Photographing highly reflective objects (especially round ones) is based on a few simple principles. The light source must be larger than the object. The last surface to be struck by the light is the light source. You are actually photographing the reflection, not the surface of the model.
Your umbrellas (assuming they are the reflective type) are not the best solution. The unbrella shaft places the light source, in this case the umbrella, too far from the subject. Even though the umbrella is much larger than the model, any round surface acts like a wide angle lens and makes its reflection look tiny.
Try this. Take a piece of paper, 8.5 x 11 is fine. Hold it at about the distance that your umbrellas would be.
Now move it in toward the model until the entire surface is covered with the reflection of the paper. This would be the correct distance for the light source. So, the larger the light source,mother further away it could be.
Transmissive umbrellas (translucent material) are aimed 180 degrees from reflective umbrellas. The light is traveling to the umbrella which is now the light source. These can be placed inches from the model.
A simple version of this is a translucent shower curtain. Cheap and big. Hang it up, aim the strobes at it and you have a large Transmissive light source.
The absolutely ideal light for small, reflective objects would be a large, translucent sphere. Model inside, lights outside, small opening for the lens.
If you have a restaurant supply nearby, they have large translucent buckets. These are perfect. Cut an opening for the lens, invert the bucket over the model and light the bucket.
BTW, the reason that an overcast day is so good for this type of photography is that the sky is a very large translucent diffuser.
I do take issue with the concept of placing diffusion material on the front of an electronic flash. The original light source,the flash tube is maybe 1"x 0.25". Increasing the size of the light source to maybe 2"x3" is a very minimal increase in light source surface area. It will help a little but not nearly to a size that would make it useful for photographing reflective objects.
Just my 2 cents but until last year I used to sign my emails like this:
Stan Shire
Professor and Department Chair
Department of Photographic Imaging
(don't miss it at all. 30 years teaching this stuff was fun, but I'm having a helluva good time making chips.)
Best
Stan