Not adjudicated
There is a charge on my bill that is not on my Explanation of Benefits. What now?
You are comparing the two documents line by line, and one charge on the bill has no counterpart on the EOB at all.
That does not automatically mean something is wrong. It means the charge and the adjudication have not been connected, and there are a few ordinary reasons for that.
The likely explanations, in order
It was billed on a separate claim. Very common in a hospital. The facility bills one claim; the radiologist, the anaesthesiologist, the pathologist and the lab each bill their own. Each claim gets its own EOB, and you may only have been sent one of them.
It has not been submitted yet. Some charges post to your account well before they go to the plan, especially where a coding question held them up.
It was submitted and rejected before adjudication. A rejected claim — wrong member number, wrong date of birth — never reaches the point of producing an EOB. From your side it looks identical to never having been sent.
It is not a covered service and the provider did not bother submitting it, having assumed the answer.
It is not yours. Rare, but it happens, and it is worth ruling out.
What to ask
Please confirm whether [service] on [date] was submitted to my plan, and if so, please give me the claim number so I can match it to an Explanation of Benefits.
The claim number is the whole point of the question. With it you can go to your plan's website and pull the EOB yourself, and either the charge reconciles or you now have a specific thing to dispute.
If it was never submitted
Ask them to submit it. Do not pay it as a self-pay charge on a service that should have gone through your insurance — you would be paying the list price for something your plan has a negotiated rate for, and that difference is usually large.
If the filing deadline has passed because the provider sat on it, that is a CO-29 situation: the plan will deny it as untimely, and the denial assigns it to the provider rather than to you.
Before you call
Check your insurer's website for claims in the same date range. Plans post EOBs online well before mailing them, and more than half the time the missing EOB is already sitting there.
This is general information, not legal, medical or financial advice, and not a statement about your particular bill. A difference between two documents often has an ordinary explanation, and the billing office is the one who can give it.