Like tmannian suggests, one of the bulbs may not be seated properly. I've found I can sometimes jiggle the bulbs a little and they go back on. Otherwise, put together a collection of bulbs for occasional replacement.
They are all maddeningly different. This is one of the downsides of daytime running lights. My Volvo dealership parts guy sometimes just gives them to me free -- too much of a pain to write up an invoice.