Did you wait to amend until the first return was done processing? The IRS should have automatically adjusted the Recovery rebate credit on the original return for you. Or was your 1040 line 30 blank? Or what did you change on it?
Normally Amended returns take 16 weeks (4 months) to process. But these are not normal times. It may take them 6 months or more. Be patient. Keep waiting.
Maybe after 3 weeks you can track it here, https://www.irs.gov/filing/individuals/amended-returns-form-1040-x-/wheres-my-amended-return-1
