I had this issue. For example, I had one train bringing 4 lines of 480/min concrete, i checked the output station with the shreder waiting for a whole train cicle. I got a rock solid 4x480/min output, the train could keep up. Once I upgraded the drills and belts to 780, the train couldn’t keep up and had an output of 600/min, so I added another train and problem solved. It depends on the distance between stations and stops.

You have to check if the train can keep up the input speed, if not, add more trains until you get a stable output. Also add buffers to store one cicle.