I'm experiencing this as well on a Win 10 machine. About 25% of the time a call comes in and it never shows up until it's missed. I've checked the pop up settings and it's enabled. This isn't consistent, it happens once or twice every 24 hours. Would reinstalling the software fix the issue or is there a setting that can be checked or adjusted to stop this behavior from happening? Is there certain firewall ports I should open other than the standards used by the machine?