Sounds like they put a SAND base underneath and its washing out from erosion.

Either that or the base wasn't compacted good enough before adding the pavers.

The base should consist of some type of crushed rock. I use #57 gravel. When you pour it into the base make sure you compact it at about two inch deep intervals so it packs nicely. Don't use round rock as it will never settle and will not be as stable as irregular gravel. Finish off by laying a 1 inch paver base sand. Level the sand and lay the paver. For small jobs you can use a rubber mallet to set the paver in place. Large jobs its nice to use a vibrating plate compressor. Sweep paver sand inbetween the pavers to keep them tightly in place.