Are there any errors in your css file?
DW relies on clean/valid code, much more than it has in the past, for many of its features to work correctly.
Run the validator at: The W3C CSS Validation Service on your css file and make sure everything is correct structurally (no missing ; or } for example).
If that comes back clean, also validate your html. I personally haven't seen an html issue cause a problem that should be coming from your css, but I suppose it's possible, you can check the html here: Ready to check - Nu Html Checker
If neither have any effect, please post a link to your work in progress. Perhaps there is something formatted incorrectly in your MQ statements that's combining your 800 pixel with the rest?
No this is not a common issue.
However I notice that you are using a 15 inch screen, as the difference between 800px and 900 px is small on this screen size that may account for one mq not being displayed. As for css designer it may be worth deleting the Dw cache as it is possible it has become corrupted -
I had a 2 parse errors and missing ';' in CSS. Its working now.
Glad to help.
Hold onto those validator links and run them first whenever you have issues like this.
DW can do all kinds of crazy things when it runs into code issues.