Was it raining when you had your bullpen?
If the balls you are using are new or have a pleather cover, it may be you have to rub up the new balls or invest in some cheap leather cover balls for throwing. I have an assortment of balls so that bullpens can be conducted with the ball that the pitcher will be using in their league. I may be weird, but I have about 200 assorted balls and I replace about 60 of my pitching-only balls every year. They get retired into batting practice balls.
BTW–this is off-topic but I have created a new-to-me batting tee drill that I call “driving range”. I line up 5 players with tees and a bucket of balls each along the first or third base line (never both at the same time–fyi–that would be a recipe for disaster) and they try to hit home runs over the opposite field fence. If you want to see some hard swings…get your team to do this drill. “Tee’em high and watch’em fly!”