stuck!! I can't get married in the sims free play!!

I am level 17 and i am stuck with the goal "get engaged" although my sims are partners and they live together. Whenever i tap on "propose marriage" the rings are not available; the rings pictures appear DARK! Though i have the money and LP needed. I don't know what the problem is?!!! I have tow more couples who are partners and not living together, when i try yo make them get married i face the same problem!!Plz help !!!!!