Tiredness in the regular players, lack of depth as we were unable to give the regulars a rest without seeing a big drop in quality, the terribly unfortunate injury to Chaplow got rid of our urgency in midfield, teams have figured us out more and close us down quicker than before and are not as open, Lambert has been out for a few games and we don't do much to change things (or we don't have the resources to make effective changes.) if our setup with Lambert as the main man isn't working, Lallana has come back from injury and not reached the same heights as he did before.
Those are the reasons why IMO.