We know that animals can reverse summon humans who have a contract with them. So, my question is: why didn't Fukasaku reverse summon Jiraiya to Mount Myoboku while fighting Pain? If Jiraiya had enough time to write on Fukasaku's back then a reverse summon could be performed easily in that time. This would end up giving Jiraiya intel on pain and leaving the Sannin alive.
Why was reverse summoning not used to save Jiraiya? Are there any limitations on the technique that prevent its usage?