I heard the game is limited to 5000 sprites, which includes anything that moves (ducks, peeps, trains, water, litter) or is a built (rides, scenery, paths). It probably includes the weeds on the grass since they grow during the game. I also heard that even puke on the paths counts toward the 5000. I have maxed out on the number of rides (255) and staff (116) in one of my parks, and still been able to have over 2000 peeps in the park.
Try an experiment and run the TRG/SAK trainer. Sweep the paths, mow the lawns, and shoot the ducks and see if this allows you to build more. It might also be because you have "too much scenery" becuase you are using items that take up 1/4 square. This could cause you to reach the 5000 sprites much sooner than using the larger scenery items.