For the relevant AY
1. View form 26AS from efiling website and check all TDS claimed are correctly reflected in it. Download it in PDF.
2. Download your submitted return and check whether you have correctly entered all TDS details in the form as reflected in 26AS and as per the TDS certificates issued to you.
Once you make sure that there is no deficiency on your part and no inadvertant mistakes in the details furnished in the TDS columns ( Like TAN of the deductor, unique certificate number etc.,)
1. File another rectification request to "reprocess" ( of course you can't attach any documents with it. That is the worst part).
2. Once CPC confirms request received by email/SMS, raise an e-grievance attaching 1. copies of all TDS certifcates and any other proof in support of your claim, 2.the downloaded 26AS and 3. screenshot of the page which says" Tax Credit Claimed is fully matched with tax credit available in 26AS"( for the relevant AY). That is what I did. A few days back I received the second rectified order by email showing no demand.). I am yet to receive the hard copy by mail.
3. If the Demand reflects in the "Work list" and/or compliance portal asking you to comply " Disagree" stating reasons and also mentioning you have filed a rectification request. ( In my case such a demand was therein he worklist initially and by the time I wanted to respond it went missing.as stated in my earlier post)