Standard austenitic stainless steel grades such as type 304 are widely used in ammonia service since they are resistant to general corrosion and ammonia stress corrosion cracking (SCC). However, these stainless steel grades can fail by SCC that is initiated by sensitization formed during heating/welding. It is common to find low levels of elements in alloys that are not in the specification. Also, copper is intentionally added at a level of 3%-4% to produce type 302Cu (UNS 30430) stainless steel. The copper in 302Cu reduces the cold work hardening rate and 302Cu is often used as a material for fasteners. The general corrosion resistance of 302Cu is similar to that of 304 stainless steel so I suspect that the low copper levels you have found in your 304 stainless steel is not the source of your failures. You do not mention what type of failure is occurring. The type of failure may give you some hints as to the cause. For example, standard austenitic grades can suffer from chloride SCC if either the gas stream or cooling water contain chlorides.