AS is 100% correct..CHANGE ROUTINE ! YES NEGATIVES HEAVY AS HELL ! Here is what you do : Forget dumbells for awhile,do this workout with a barbell,a spotter, and 1 time per week.
Find your max weight on 12 reps flat bench barbell.Start with that weight and add weight as you go down in sets in this order ; 12,10,8,6,4,2 you will be adding 10-20 lbs each time.Take 2 to 2.5 minute rest between each set.Now strip weight as you go back up...4,6,8,10 ,12...you can do 2 different tri exercises at this point and the chest day is over.If you do this correctly for 60 days you will increase your bench.As the weeks go by you will have to start off with more weight as you get stronger.It is a failure workout...if you get through the entire workout every set every rep you have to add weight the next week so at some point you begin to fail..this is why you have to have a spotter.At end of work out every other week load the barbell with 10-20 % more weight than your 2 set and do a few ( 4-6 ) negatives...OUCH !!!!!!