I have made similar experiences with Chartboost which sometimes left me confused and just went away a few days later.
Maybe this link is helpful for you: https://answers.chartboost.com/hc/en-us/articles/201121969-I-can-t-see-any-ads-in-my-game
The thing is that (from my personal experience) Chartboost just isn’t always able to deliver for all types of ads. What you could do to handle these cases is to add e.g. AdMob (http://v-play.net/doc/plugin-admob/) as well, and display an AdMob interstitial if Chartboost fails to load. Also, I’ve heard we might have a second Plugin that offers rewarded videos in the pipeline, so stay tuned 😉