chia plots check -n 30 to try 30 sample challenges for each plot. Each of your plots should return a number around 30, which means it found around 100% of the attempted proofs of space. If you're still worried try
-n 100 as more random attempts will give you a more valid assessment that the plots is fine. It really is ok if your plot is within 80%-120%. If some of your plots are missing for some reason you may need to add the directory they are in to your config.yaml file. That can be done in the GUI with the MANAGE PLOT DIRECTORIES button or on the command line with
chia plots add -d [directory].