The adds phase has two kind of dps-race mechanics.
One would be: Killing adds in time before Ramuh decides to cast Judgementbolt
The other would be: Adds cast "increased sentence" on set intervals. This adds damage to judgementbolt. Unless your group is wiping because they're unable to kill all the adds in time, this is the most likely reason.

Some would think Summoners would be godly during the adds phase. In a sense, yes. But with the second mechanic of the adds, too many summoners will actually wipe the group. You'd want to burst them down one by one as quickly as possible, rather than killing them together in a short time span. It's not rocket science to realise that 6 adds that uses increased sentence would give Ramuh more "stacks" than 5 adds (and so on).

As a summoner, you'd want to throw DoTs on the first add, work on the second while festering the first. Also micro your pet that it attacks the add in order, rather than have it "do it's thing". Do keep casting DoTs on the adds, regardless of where the party is on. But save big burst abilities (Enkindle and Fester) for the add the group is on.