I'm experiencing the exact same problem with my 9 week old. He used to be a brilliant sleeper during the day, even allowing me to have a sleep, but getting him to sleep during the day now is such hard work. Sometimes he goes straight down, and other times wakes up screaming after 20 minutes and will only sleep in my arms or on my chest. Is he on a routine or are you demand feeding? I was trying to stick to a 4 hourly routine, but just today have started to feed on demand more to see if that makes a difference.
He has just started to sleep through the night (well for the last couple of nights from 10ish-6.30) so I guess I can't really complain too much, but it is frustrating when they won't sleep during the day when you are trying to rest yourself or get things done.
What I've said probably hasn't been much help, but I do sympathise with you, and will let you know if I come up with any magic solutions. smile