Coming from a background of thirty years in multiply committed relationships (since sharing with my first water brothers in high school), I've seen and experienced jealousy in many forms.
Time jealousy is commonest even among stalwart supporters of polyamory. In monogamous relationships, this may take the form of jealousy over a spouse's job or the amount of time she spends with her mother. That kind continues to exist in our lifestyle, but we add to it jealousy over which lover/mate is getting more alone time.
Which one is getting more showers together, who gets Saturday night? One answer is to do as much as possible with both at once, if this is a triad.I used to recommend this unreservedly, until getting into a situation where I actually wanted more alone time with my primary partner (we were newly weds) than with my live in lover, then handfasted to us. It was a funny situation. We were a triad at the time, but looking for a lady he could marry and have children with who would have been part of our marriage, too, had it all worked out (it didn't, but he is now happily married to another of our water siblings.) Maybe part of my focus of attention was a way of protecting myself from his leaving with the lady he found (as he did, though both are still close friends of ours). Perhaps we would have stayed together as a group marriage had I given him more loving attention. I'll never be sure, but I think I'll do it differently at our next opportunity--live and learn.
Another kind of jealousy is the kind most likely to be based on insecurity about oneself as lover or as a person. This kind of jealousy occurs when you worry your primary partner will leave with someone who is better looking, smarter, better in bed, etc than you are. In an open relationship, this can lead to the constant testing of who is more important, you or that other someone your primary is seeing. It can take the form of last minute illnesses before a planned date and other ploys requiring a choice. It being an axiom that you should be "ok" with your primary before giving attention elsewhere, picking fights before such a date fairly regularly is another symptom of this kind of jealousy. The person doing these things may not be doing them manipulatively, but may be subconsciously setting up situations to reassure them of primariness. Such reassurance is better provided, by plenty of physical affection (including sex) and verbal reassurances of love all the time, but a little extra before seeing another lover. If that doesn't do it, and the behavior continues, marriage counseling is certainly in order and probably some individual self-esteem therapy for the jealous partner.
Be sure to pick a therapist experienced in such counseling or at least comfortable with your lifestyle. Any prejudice here could be anything from a waste of time and money to a way to break up your marriage. Ask questions first. Get a referral from someone you trust, if possible. If their is a regular other lover involved and certainly if that person lives with you, they should be included in some of the counseling. If the counselor says no, DON'T go to that counselor.
Jealousy can happen even in the nicest polyamorous families. The difference between us and those frozen in monogamy is that we know it's a problem and not something for which there is righteous justification. As long as that's understood, cut your jealous partner or your jealous self some slack while the problem gets handled. This needn't mean a break up, particularly in the case of a committed secondary relationship. It does mean plenty of caring communication and some willingness to adjust for the comfort of all.