This is a problem I have noticed for a while now and it is something I really want to improve on.
I would not classify myself a hard smasher and I believe this is a part of my game that is a huge disadvantage for me. Ideally, I would have a stronger smash as I always find myself not being able to kill off the rallies.
Basically, my smashes are more powerful when I am smashing at an angle. This is illustrated in the diagram below.
I cannot generate as much power when I smash straight down. What may be causing this and how can I fix it? I believe this is related to why my smashes are generally quite weak.
I can also smash harder when I am on the left side of the court (I'm right-handed). This points to a technique problem.
Get a coach to watch you. How can we tell what is wrong with your smash from court diagrams!?
If you can't however, then I suggest looking at your grip, set-up and overhead stroke. I had a similar problem where my body was already too open hence why I couldn't generate maximum power on a straight line smash. Post up a video?
You may be swinging out of line. In other words, the racket head isn't traveling parallel to the direction you're hitting to. Because you're slicing slightly off the string-bed, you're not getting the full benefit. When you hit at an angle, the direction you're hitting the shuttle may be closer to parallel to the racket head velocity, hence you get more speed.
Also, you may be too far forward when you strike, preventing you from getting a sufficiently steep smash.
As has already been stated, all of this is pure speculation without a coach watching or you taking a video of yourself. When you video yourself, get some head-on examples as well as side-on. Also, video 'straight' as well as angled hits to compare and contrast. If there's a balcony above/behind the court, some overhead angles could be useful too.
Perhaps try turning your body 90 degrees to the net (sideways) and then twist your hips into the shot. What direction your stomach faces at the point of contact is where the shuttle will go, unless you slice.
I believe that the reason you are able to smash harder at angles instead of straight smashes is because you have the time to use body rotation. Or even, you have more time to hit the shuttle than when it is at your corners. It should be possible to make your smash ever harder if you can get to the corners faster and get a good body rotation along with your regular smash.
PS: Don't force yourself to get to the corners and then rotate your body for the smash you will need practice otherwise you might injure your leg when landing or something.