Coucbase 3.X Objects not found

I used couchbase db for sometime and came across that objects get deleted/lost after sometime and what could be the reason.

Two scenarios that could lead to this are are either failover without implementing durability requirements at the application level or setting an expiration on the items. Is there anything more you can describe about the environment?

Thank you very much for your kind reply. We have 3 nodes on centos 6.5 linux environment. We are using couch base 4.1. Our Application is written on .net and the client library we use is 1.3.10