A quincunx (or inconjunct) happens when two celestial bodies have an angle of 150° using an orb of 2°.
Paradoxically, this aspect connecting two astral items indicates an absence of connexion between their energies. It is like oil and water, they don't mix. It's about not connecting the dots, an incapacity to understand or see what might be obvious to others. It's disconnection. In a personal chart, it should be interpreted as seclusion, Jupiter operates independently from Venus, for example. It's one hand does not talk to the other.
In synastry, both parties notice each other for sure, but their respective energies don't know how to connect. The chemistry does not take off. If they don't have other favorable aspects between them, partners may wonder why the relationship does not work, why it does not come naturally. There is a permanent lack of understanding.
Unfortunately, it is not a situation which can be resolved with adjustments. There is a fundamental impossibility to find commons grounds (unlike for oppositions). The solution is to agree to disagree. Tolerance and concessions help too. You need to accept the situation as it is: imperfect. Don't try to solve it, work around it. Learn to live with it, if possible. If that is not possible, then have no regrets, it was not meant to be.
If the astral items are involved in conjunctions too, then there may be possibilities to create agreements through negotiations and discussions. For personal charts, meeting someone who has the same astral items in Trine will tell you where and how you are not connecting the dots. It's your antidote. You may experience ha-a moments.
Quincunx teams with semi-square to achieve their deeds. They also aggravate oppositions and can push squares into total meltdown experiences. However, quincunx aspects are not always negative. Say you have Moon trine Neptune, you escape into spiritual experiences and your emotions flourish inside your imagination naturally. You are not well connected to reality. But, if your Moon or Neptune is quincunx Venus (for example), your friends or your partner will help you set boundaries between what's real and unreal in order to restore balance.
Related aspects: sextiles and yods
Other minor & rare aspects: grand-cross, grand trine, quintile & biquintile, semi-sextile, semi-square, sesquisquare and t-square