I am running a campaign using
What is that?
Cookies/cache. Give it a few minutes and it will come back.
It happens. Also when you open campaign report the numbers could be different. It need some time (usually few minutes as servandosilva said) to get the right numbers, so keep that it in mind and not to fast decisions. 
Hi guys,
We are using an “eventual consistency model” and sometimes (rarely), when there is a high demand on our servers, you may see stale data for up to 2 minutes. We will be working on a fix for this, and we will update you once this is settled.