kts64s - I usually only feed Henry on 1 side now, takes him 10-15mins. When he falls off I burp him, wait a few minutes then offer the other side. I don't force the issue - if he's hungry he might go on for about 5 mins, otherwise he just shows no interest. I haven't had any issues feeding either side though. Do you try him on the left first?
I agree with Sterla on the tiredness thing - Henry can be a bit fussy if he's tired too.