A likely reason the exchange failed is that the RI modification took too long to complete.
AWS has an SLA of 4 hours for RI modifications to be successfully transacted and the new RIs to show up in the customer's inventory. Accordingly, the exchange automation will wait 4 hours and 15 minutes for the modified RIs to show up before marking the exchange as failed and aborting. If the new RIs show up later after the 4 hour window, the exchange will not be continued.