I don’t know who/what company made the mould, but it seems to me to be an innapropriate mould. unless it was supplied with a first stage mould. Deep moulds usually need a shallower 1st slump mould before putting into the second, final stage mould. Slow slumping – as you did – usually helps and so does low temperature slumps – they take a long time though. Although I have been slumping for a long time, I would not attempt one like this even now.