Sims Relationship bar Keeps Dropping

Hi, I hope someone can help me solve this issue i’ve had for a while. My sims’ relationship bar keeps dropping and turning red and I’m not doing anything to make it be that way. I have to build up their relationship all over again, and when I leave for another family and come back, it’s dropped again. Do I have to continuously be playing as this couple to keep their relationship together? It wasn’t like this before LoveStruck. ;(