Dips are great, Flat BB or DB presses, Decline is ok but IMO it hits your anterior deltoid more than anything.
Its good to think of your muslces as a balloon, they dont chage shape. They just get bigger, if you have a really flat/small lower chest and nothing you seem to do fixes it you may never get a really full lower chest. Look at Craig Titus as a great example of someone who takes "supps" and presses a lot, but has no lower pecs what so ever.
The Pectoralis Major is just one large muscle, its points of insertion are the sternum and clavicle. It is possible to put the point of tension on these different areas, i.e. Incline Presses or Flat Presses. Ultimately, any press you do will work the entire muscle. If you have problems building your lower chest try building your upper chest to help bring your overall chest development up.
What is your current routine like?
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I agree with Braaq about the dips, they work the lower part of the chest good. But the delts even more so imo. Bringing the overall chest size up seems to be the easiest solution.
If you aren't doing it already, start doing incline benchpress/dumbbellpress.
But not with to high angle, if you can handle it.
Then start to do dumbbellflyes going for contraction instead of stretch, meaning, dont drop the dumbbells to far down (3/4 of parallel)
Then stretch on a wall bar. Grab a bar at shoulder level with each arm (facing away from the wall bar) then squat down and erect your chest to get a full good stretch in the chest. (It might hurt a bit in your elbows afterwards)