Why is my flat bench press stalling?

For starters: 5' 7 135lbs doing PPL routine. 6 Weeks ago I was at 130lb and my bench press was 115lb(5reps)

I went to 3x8 about 3 weeks ago Flat Bench:100lb Incline Bench: 75lb Decline Bench:85lb

Today (3x8): Flat Bench: 115lb Incline Bench:100lb Decline Bench:110lb

My incline and decline have been increasing normally adding 5lbs every chest day. My flat bench though has stalled not increasing for almost two weeks.

To add to that it also seems to be the trend that barbell lifts are significantly higher than dumbbell lifts for people, well for me my barbell and dumbbell flat bench there's only like a 5lb difference. What could be the cause of this lag?