IIRC you must wait at least 2 weeks (14 days) after you take the flight for the miles to be credit. After the 2 weeks are up, you should be able to use the form no problem. A colleague of mine had a similar denial issue as you but after telling them to wait 2 weeks they said the flight credited no problem.

Whenever you fly you should check your boarding pass to see if the frequent flyer indicator field is filled in. In addition, during IRROPs I would recommend getting a paper boarding pass and keeping it until the flights credit since frequent flyer details can sometimes disappear. If you want to do a mobile check in, I'd recommend taking a screenshot of the boarding passes as evidence. Additionally, as UA collector (with no status) you should see UA somewhere on the boarding pass where the frequent flyer field appears, which confirms that your frequent flyer details have been accepted. When you gain status with UA it will then say the relevant status (i.e. UA*G for myself or UA*S if you hit the Silver milestone). Remember, you can add your frequent flyer details at any point up to the doors closing on the airplane (i.e. you could add it at the check in counter, at the gate, at the lounge, at security at some airports, etc.)

Safe Travels,

James