A monthly plan is a subscriber based plan and each plan has a subscriber limit. If you are unable to import any more addresses then you most likely hit your limit. You can check to see if you reached your limit on the dashboard.
Here are your options:
You can upgrade to the next higher plan depending on the new list you want to import.
For an example if your current plan is on Level 5, which allows you to upload 10,000-24,999 subscribers, and you have already imported 24,499 subscribers and you would like to import a new list that has 15,000 more subscribers - you would need to upgrade it to Level 6 which will allow up to 49,999 addresses. Plan upgrades will be prorated based on the day of the upgrade.
Another option for you is to remove subscribers that have not been mailed in the last 30 days. If you have mailed them on the last 30 days the system will not allow you to delete those addresses. By deleting inactive subscribers this will save you more space for subscribers to be imported.