In the many school where I conduct sectionals, some students tell me that their embouchure isn't strong enough or that they don't have enough endurance. I ask them if they are playing every day and the answer is almost always: "well..."
It's sound like a no brainer but you need to play almost every day to build a strong embouchure and develop endurance. Ideally, you should be playing your instrument at least 6 days per week by yourself or in rehearsal. You can't expect much if you only pick up your horn for band practice three times per week. If you already know your band or orchestra pieces well, practice some solo pieces or etudes. If you are satisfied with only playing in the band and don't wish to invest more time and effort in your playing, it's your choice. That's fine but since brass playing requires some strength and endurance at some point, you might want to put a little extra effort sometimes if you're having trouble keeping up with the group. At least a few days before a performance.
I am associate principal horn of the Singapore Symphony Orchestra and the author of the progressive methods. I'm happy to share my experience as a horn player and teacher with you.