Peoples grinders are probably close enough together that they are 'stealing' spawns from each other. Either that or you've got a lot of 'honeypot' tunnels/caves nearby stealing spawns from your grinder. Explore and either light up or fill in all caves in a (minimum) 32 block radius from your base. I'd probably go with 64 for good measure. Technically they can spawn out to 128, but after 64, they don't really move around and tend to despawn naturally in fairly short order.
Of course, that means also making sure it is at least 64 blocks away from the nearest other player's mob grinder as well...
Furthermore, there's a hard cap on how many mobs can be spawned in at any given time, so if there's more than a couple of players, you may be hitting that hard cap.