I'm on several of the PODs you listed above, including Zazzle, which is the one I've been using the longest (about ten years).
In the beginning Zazzle was great and I made sales regularly. However, as the marketplace became more saturated, Zazzle introduced new measures to their advantage, making it harder for smaller designers to get noticed.
Zazzle has always allowed creators to set their own royalty however if you set your royalty above 14.9 %, your product will not show up in the marketplace. This means if you want potential customers to see it, you'll have to promote it on your own site, social media, etc.
Zazzle introduced something called Zrank which is a grade of 1 to 10 you receive on your Zazzle store (10 being the best). The more you keep your store updated, the better your score will be. Stores with high scores do better. This is easier said than done. Products with similar titles and tags, lower your score. It is the worst thing they've implemented, in my opinion. You are constantly chasing your tail, especially if you have more than one store.
Zazzle will hide products in your store if it is not selling or hasn't been viewed in a while. You can fix this by freshening up the products description or tags which will make it visible again. This is just one more thing that creates more work for you than its worth.
All of this has meant dwindling sales for me which is why I've opened stores at other POD sites. most recently at TeePublic.