Was this liquid yeast or dry yeast? Liquid yeast will lose vitality if over heated and not having wort sugars to eat. A starter should be made with liquid yeast even if properly handled and moderate OG worts.
I wouldn't worry too much if you don't see bubbling through the air lock when using a bucket fermentor. The lids often don't seal well letting the CO2 escape around the rim until the fermentation becomes more vigorous. This is not a problem. Some buckets are designed this way and do not have provision for an air lock.
You can pull the air lock and look for krausen formation. Sanitize the area first. You can also hold a bright flash light to the opposite side of the fermentor to see if there is a foamy krausen forming above the liquid.
Let us know if you do or do not see a krausen forming.