At the moment of sending the campaign, the emails arrive to the clients but if the client opens it, it is not being marked in the subpanel seen, nor in the statistics of the campaign
This might not be so simple, see
https://github.com/salesagility/SuiteCRM/issues/1521
read the comments from 18 Jan and later.
Basically, some browsers won’t show images, and the loading of images is what gives SuiteCRM the information about the email being opened. You can test that in Gmail, if you click to show images in the email.