GST charged by my bank on a forex conversion isn't showing in my GSTR-2B. What can I do?

The credit appears only if the bank reports the invoice against your GSTIN in its GSTR-1 for the right period. When it doesn't, push for a correct invoice and correct reporting:

1. Gather three documents: the remittance advice (amount, rate, date), the bank statement showing the GST debit, and your GSTR-2B for that month showing it missing.
2. Write to the bank: give the transaction date, GST amount and your GSTIN, and ask them to issue the tax invoice and report it in GSTR-1 (or amend it if it went to a wrong GSTIN, or was filed as a B2C invoice without one).
3. Check any replacement invoice carefully. Banks sometimes issue a fresh invoice dated months later for the original deduction. The invoice date should match when the GST was actually collected, and your statement proves that date.
4. Verify that a corrected invoice is genuine. Check its QR code and details against the GST portal before relying on it.
5. Escalate: the Principal Nodal Officer, then the RBI Ombudsman, as a deficiency in service. Keep it short and lead with the documents.

Your own records are your strongest evidence. In my case, the remittance advice, statement and GSTR-2B together proved when the bank had actually collected the GST, contradicting the date it gave.