No personal experience with this situation, but given that you're still in the "lag phase", you may want to aerate the wort or inject some O2 into the wort to encourage further yeast growth. Sometimes brewers do this (aerate or oxygenate 12-24 hours after pitching) for high gravity worts. I don't know if this is a good idea for underpitched yeast. It is what we do when we are doing yeast starters, though. Someone can come along and advise as to whether this is a good idea or not.