Well, in the category archives sections, the excerpt “more” won’t work unless you insert it after the first 55 words of the original post.
There is a custom function you can use which will allow you to reduce this number… but it will occur on every post in every category archive section.
So, you have two options for this area…
1. Add add least 55 words before you insert the “more” tag.
2. Use the function to reduce the number of words, which will affect all posts in the category archives sections.