Why won't the van start with 4months old battery after 2-3 days without use

Van won't start after I leave it for over 2-3 days without used, I bought anew battery it was alright for afew weeks but now it doing the same thing when I go to start the van after 2-3 days it just goes clic clic clic and the moter won't start I have to jump start it with leads, the van is 2005 ldv maxus 3.5 CDI lwb