WLM on Blackberry Curve issue

[grumpy]

Hey guys,
I've been having some issues on my BBerry Curve with WLM. It stops receiving IMs from my contacts, they don't bounce back to them and my IMs go to them without problem. It happens sometimes during conversation, one moment all is ok the other no answer or for example I leave it on during night and someone sends me IM in the morning and nothing comes (but not always). When I sign out and back in I can receive IMs again but not of the old ones ever come. Is someone having similar issue? Is there any help for that and why does that happen at first place?

Help, please!

Thanks!