If you've created a custom field and data entered in that field won't save you may need to give the server a little bit of time to catch up to your changes. Leave it alone for up to an hour or two and then try again. This should resolve the issue most of the time.
If the data still won't save then the solution would be to delete the field and re-add it.
Here is an example scenario:
Let's say you've added a custom field on the contact form titled "Office ID" and once added, you attempt to enter text in this field, save the contact record form, re-open it and the data that you've entered is no longer there. The solution to this intermittent issue would be to delete the custom field - not just from the form itself - but the entity by way of the following steps:
[for the above-mentioned example we'll work with the contact entity]
First and foremost, you'll want to remove the field from whatever form you've added it to [i.e. the Contact form] by opening any contact record, clicking on the Form button (either on the toolbar or under the More Commands button), clicking on the field in question that you wish to delete within the form [wherever you've placed it], clicking the Remove button at the top of the page in the toolbar, Save the record, Publish the record, and Save and Close the record.
1. Go to Settings > Customizations > Customize the System
2. Select the Entity
3. Expand the Entities section on the left-side menu
4. Expand the Contact section on the left-side menu and click on Fields within that sub-section.
5. Locate and select the check box next to the custom field in question [will likely start with "cobalt" - example: "cobalt_officeid"].
6. Click the delete button at the top of the page [black "x" in the toolbar].
7. Recreate the field.
8. Save Changes and Publish Customizations
9. Add field back to form.
10. Try to update (then wait an hour or two if it does not right away)
*If after going through this process you still can't update the new field contact support.