My Kolsch was slow to begin even when I pitched a decent starter (48hrs w/o and 24hrs with).
However, if you are using a plastic primary make sure your seal is good. I have a Double IPA that I made last week and pitched a 3qt starter into. 24hrs later I had no airlock activity...popped the lid and found a huge krausen. I resealed the lid and the airlock started burping away. It was just a bad seal. It had been fermenting away and the CO2 was finding a different path out.