Spritesheets still count as individual sprites. Each weapon handob consists of 8 sprites, so if you put a handob at 915, it will use up 915, 916, 917, 918, 919, 920, 921, 922. If you later put handobs at 916, 917, 918, they will all overwrite the previous ones.
So you need to space out the IDs appropriately (eg. 915, 923, 931).