I’d wait two more months
I am assuming you’re getting your credit reports online and not ordering the actual hard copy from the bureaus.
The online services are normally about 90 days behind on their reporting. If you have only had the card for 4 months that would explain why it is not yet appearing in your online reports.
As long as you are using the card I am sure Capital One is reporting to the bureaus. Just be patient.. I am sure it will show up on Experian shortly.
I’d wait 2 more months (you need at least minimum of six months to have a score anyway), and if it doesn’t sort itself out by then I’d call or write Capital One.