I’m not seeing the star adjuster or the spring on the bottom. I’m assuming the axle is blocking it in the picture.
Here’s a picture of mine, also a ‘51 3100. I’m not saying that this is your problem, but one thing I had some problems with installing the shoes, was the shoe retainer on the shoe anchor. The retainer slides over the shoe anchor and sits on a shoulder. When I tried to install the shoe return springs over the anchor, the retainer would slide off the shoulder. You can still install both springs but the retainer is not preventing the shoes from sliding toward the drum and opens up a gap large enough that might allow the condition you are finding.
Phak1 your photo shows you have the primary & secondary shoes reversed.
