Hi Jerome,
By valid, I mean that every record for which you want to display a marker needs to have all the data that the markers display needs. I my case I had a set of records with city, state, country information. For few a locations the result of the Google API call was empty resulting in, for example, the city being missing for a few records. If this happens none of the markers will show.
Start by getting 1 or 2 markers to show. Then try to get more to show at once.
Also, see my response here: Anybody used maps? - #29 by dongahgan