The Link below leads you to IRS form 3911. It says for Tax Refunds, but it is also to track missing stimulus payments (EIP).
Please follow the instructions below if you are sure you qualified for a stimulus check but have not received it yet.

First, make sure someone else did not declare you as a dependent.
Do look for your payment in the same account where you receive Social Security or SSDI, or on your SSI card or at the mailing address or bank account where you filed your 2018 or 2019 tax return. If you moved, or changed banks, the stimulus check would have been returned.
You’re not supposed to ask for a trace until you receive the required letter letting you know where your check has been sent or deposited. But this is taking too long! (The IRS has until December 15, 2020 to mail your stimulus, according to the law.)

Update: The below is correct, but they have created a special toll-free number to call re missing stimulus payments. So while you should also file form 3911, try calling 800-919-9835
The instructions below start a “trace” on your payment.
There are no guarantees they will respond to filing this form – and it may take weeks to get a response, but at least you’ll feel better knowing you’ve done something!!
Also,I have highlighted the mailing address for Illinois Residents.
Below is an exact copy of the information I received from the IRS.
Terry
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A trace on an Economic Impact Payment (EIP) follows the same process as a trace on a tax refund. You will generally receive a response 6 weeks after we receive your request for a Payment trace, but there may be delays due to limited staffing..
To start a Payment trace:
• Call us at 800-919-9835 (you may experience long wait times or recorded help because our staffing is limited)
or
• Submit a completed Form 3911, Taxpayer Statement Regarding Refund (PDF).
Here is the direct link to that form: https://www.irs.gov/pub/irs-pdf/f3911.pdf

Follow instructions below – and if you live in Illinois, mail the completed form to:
Philadelphia Internal Revenue Service
2970 Market St.
Philadelphia, PA 19104

• If you submit the form and you are Married Filing Jointly, both spouses must sign the form;
• Write “EIP” and on the top of the form and complete Sections I, II and III;
• Answer the refund questions as they relate to your EIP;
• When completing Number 7 under Section I, check the box for “Individual” as the Type of return; enter “2020” as the tax period and leave the date filed blank;