A common occurrence with heterosexual poly couples is unbalanced success in polyamory. Not that it can’t happen with couples of other orientations, I just have observed with heterosexual couples more commonly.
The most frequent instance of this is when a monogamous couple transitions to polyamory. In one occasion, documented over on /r/polyamory, is what appears to be a heterosexual relationship (based on the attention given to male identifying terms) between a man and a woman. His girlfriend had no trouble finding additional partners. While he, on the other hand, can barely get more than a first date.
In the five years that I have been practicing poly, this is a situation I have been in. My female partners have typically been able to outnumber my successes finding new sexual or romantic partners. It’s perfectly normal in poly, and this can go back and forth over the course of relationships.
While it is perfectly normal for this to happen, it can lead the less successful partner to feel insecure. As with any successful poly, navigating these issues/circumstances involves communication. However, it is mostly the insecure person’s responsibility on processing these emotions. Not to say one couldn’t request reassurance from their partner, but it is also not their responsibility to process that.
When I encountered this, I overcame it with self-improvement. Going to the gym, getting out to meet new people, rolling with the first dates that might not go anywhere. The big thing is to keep trying, to put yourself out there. If one starts to succumb to those insecure feelings, you get stuck in a cycle of insecurity, and no one finds that attractive.
Sometimes the poly experience isn’t balanced. But if you can push through the unbalanced times, your poly will become stronger.