Why does our Sub-User's form have limited submissions?

We recently created a sub-user for our account and they are showing that their form is only allowed 100 submissions. Through our account, we are allowed 1,000 monthly submissions. 

Is there a way we can share the allowed submissions?

The sub-user feature is for you to share your forms with someone else. This person's account is a separate account.

You are not inheriting your account benefits, you are simply granting access to your form or forms, depending on the privileges you give to each sub-user: https://www.jotform.com/help/232-How-to-share-forms-with-a-sub-account-user 

If the sub-user creates a form in his account, that form will be subject to benefits his account type provides. 

If you want the sub-user to create forms for your account, he will need to do it under the folder(which label is your username) that is located in the "Shared with you" area, example:

Let us know if you have more questions, we will be glad to assist you.

So we would need to create the form they're wanting to utilize in our main account and then grant them access?

Correct, but If the sub-user has already created a form in his own account, and you want that form to be subject to your account benefits, then the sub-user can transfer ownership of the form to your account, simply by dragging and dropping the form into the folder(which label is your username) located under the "Shared with you area":

Please check this guide to see the details on how to do this: https://www.jotform.com/help/342-How-to-transfer-or-share-forms-and-data-with-another-account 

Let us know if you have more questions.

Great, thank you!! I will have our sub-user give this a try and we will let you know if we have any more questions.

Thanks again!