If your prints look ok in room or daylight after complete processing, and stay that way for a bit, THEN at some point turn yellow, it is definitely due to a lack of proper fixing or complete removal of the fix...

Please let us know in more detail your exact processing steps, and the chemistry, dilutions, and times you are using. I mention this because photo chemistry works in synergy, not seperately. IOW, proper fixing requires a good stop bath before, correct dilultion with two seperate fix baths (with a rinse in between), then hypo clearing agent and a complete wash afterwards...

BTW, there is no such thing as fixer solution that is too fresh...