some feed links don’t work recently….

some feed links work correctly, but some feed links don’t do at all.

Firstly I checked whether other plugins influence or not by inactivating other plugins one by one and all plugins other than Feedwordpress.

however, maybe they don’t seem to influence Feedwordpress plugin.

Secondly, I changed wordpress theme from original to default theme but the situation is same as before.

What should I try next? also what causes this bug?