That's very strange. Have you been able to open an issue with Atlassian Support? The messages are sending but your own client isn't seeing them received...
Usually most users can fix this issue by fully closing the HipChat client and then reopening it. This fix doesn't work for you?
...have mention name FirstnameLastname. Fe.: @MaartenCautreels This certainly helps to know who is being mentioned in a conversation. Imagine your colleague sends the following message...
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