@medom Sorry for the trouble! There are a couple of things that could cause this - the first thing that comes to mind are locked dates for the tasks that are not moving. If locked dates are the cause of the problem, clicking the small padlock icon in the Task Inspector to unlock the dates should resolve the issue.
If this doesn’t resolve the problem for you, our Support team would be happy to take a look at your file with you! They can be reached at omniplan@omnigroup.com. If sending the file in is an option for you, please compress the file in Finder before attaching it to the email (to do so, right click or control click on the document in Finder and select “Compress” from the contextual menu).
If you’d prefer to send in an anonymized copy of your file, you can do so by selecting Help > Contact Omni… in the OmniPlan menu bar and selecting the option to include a copy of the file. This will create an email with a copy of your OmniPlan document, replacing all of the text in your document with “Xxxxx” strings.