Teens are going through puberty and handling a lot of first time social and/or romantic/emotional situations while they are also trying to find themselves amongst their peers. Most teens are not completely honest with their parents (I know I wasn't)... If you have a good relationship with your kids, a straightforward conversation about how they feel about spending time together can be eye-opening. What don't they like about it? What would they rather do? It's tough for them to have to choose between a social activity with their friends and a family activity.
As a parent, being open to compromising and planning family events around your teen's preferences can also go a long way. Are they into outdoor activities? Music? Cooking? Allow them to participate in the planning as a way to get to know them better. Allowing them to participate regularly but less often can also be a game changer. Give them the option to choose how many times per month it happens. Maybe it will feel less like something they have to do and more like something they want to do. Also, don't call it family night.