If I understand you correctly, you bewed some quantity less than five gallons, poured it into your fermentation vessel with additional water to make the requisite amount, then pitched the yeast. If that is correct, that is a good general procedure, although there are a lot of details omitted. The yeast should have taken off within 30 hours and started fermenting.
Are you using a bucket to ferment in? If so, is the lid sealed really well? If it isn't, the C02 can escape somewhere besides out the airlock. Take a peak and see if you see any kraeusen or a kraeusen ring that indicates that fermentation already took place.
If that doesn't lead you to an answer, please gives us a more detailed description of the beer you were brewing, the ingredients used, the process you followed exactly, etc.