MSMQ Workgroup - Queues contain messages, but I can't see any

I've been using MSMQ for some time but found this issue to be really weird.

We have a GPS tracker which sends messages to MSMQ. I made sure the queues I'm using are not empty, programatically, but when I check the queues manually via Services & Applications > MSMQ, none of the queues show any message. First I thought this could be related to permission, but I've logged with an 开发者_开发知识库administrative user and the problem continues.

Any hints?

Thanks regards,