I had the same trouble and took some lessons from Tony Marcino...
I had an "all-arm" break that was around 17 consistently, but very seldom went over 20.
His advice helped a lot. Basically if you can't get more mph (with control) doing a simple motion
like the spf-style all-arm break...you need to switch to a more athletic motion that involves a bigger swing,
more of the upper arm, and some of the body weight too.
So, longer bridge might be needed. Bend your rear leg so that you can spring/lunge forward off of it.
Keep the back arm bent a bit at the elbow because when you stand up, it will naturally un-bend a bit.
As you stand up and lunge forward, you gotta time the forward swing of the arm
to match the forward motion of the body.
The tip should make contact when the forearm is vertical, not when it's past vertical or not quite vertical yet.
Too early or too late and you lose power, so that's the 'timing' thing people refer to.
Trying a big arm swing while the whole body is in motion is difficult, it needs more coordination,
it's like hitting a baseball. Lots of moving parts. You need to practice at low speed and NOT try to hit 20+ mph,
until you can get the timing and lunge part down, then gradually build power.
It's always been very hard for me to resist trying the shot at full power but it's necessary.
Keep in mind that not hitting the head ball full directly costs you MPH in terms
of how the balls spread. So a half-ball hit at 20mph only put half the energy into the rack, and the rest
is wasted on making the cue ball fly around uselessly. It's no better than full hit at 10mph.
So focus on the full hit above all.
Lastly, if you need to make balls right away (like it's a tournament) and you can't get this power break working,
learn the "trick breaks" where you play to make a specific ball on the break. The wing ball in 9 ball,
or the 1 in the side... the 2nd row balls in 10b, or the back corner ball going 4 rails into the corner. And in 8b,
the 2nd ball break that tends to send something rocketing into the opposite side pocket, or into the near side pocket.
These all require a tight rack but can be done at lower speeds.
(Maybe not the 10b, I've never been able to make those 2nd row balls without some power)