Displaying a HUD message in client_PostThink is a very bad idea. This will display the message 100 times each second. Since there can be only 4 HUD messages on the screen at a time, this will of course overwrite every other message from any other plugin, especially if you're not using ShowSyncHudMsg. I doubt you need to display the message that often. The least you could do is use a repeating task of 1 second (or 0.1 if you need it more often). Also make sure the messages are displayed on the same channel. Bear in mind that this isn't possible with DHUD.