Ok, idea is simple:
Not so much people today use howitzers in game, and that's reasonable. It usually reqiuers to take s**ty doctrines (like god-hatred Royal Artillery), they cost a lot of MP, eat a lot of pop cap, and they works mostly awfull. B-4, Sexton, QF 25, and even Priest! Only ML-20 and LeFH can do something real, but they are actually bad too. They just don't worth all invests, which you should pull in them.
But... Rocket artillery, like LandMattress or Katyushas performs perfectly! They just wiping out entire squads and buildings in huge areas and do that really fast! And that's what real artillery should do. So, somehow we get, that rocket artillery today is way more usefull as artillery, than Howitzers.
"So, what's a problem?", would you ask. And I will say - all howitzers in game are doctrinal (QF 25 is not howitzer, it's decoration for my base, just like flowers *
*) and mostly static. Meanwhile, Katyushas, Werfers and Frammens are able without any doctrines and all are mobile. So, they are not only more effetive, they also have better survivability!
Of course, howitzers have bigger range of fire and have penetration to deal with tanks, but... For to kill tank with howitzer you should be damn lucky, cos it requiers direct hitting of vechile (rarely happens, actually) and not just 1, because direct damage is not so high. So, Howitzers as long-range tank killers doesn't work, B-4 did it well before, but now it's only good to shoot at Hitler's mom ass somewhere in Berlin. And even that big target it will miss...
And bigger range is not such great benefit, because... rocket platforms can move closely and not only get to requiered range of fire, but also make accuracy better by moving even more close to target!!! It's twice more cool, than howitzer's "long range". And of course as I said - mobility grants survivability. Such horrors as Dive Bombs of PTAB attacks are not dangerous for them.
So, I can only suggest here pretty "weak" way of fixing that - change rocket and howitzer artillery places. Make Howitzers undoctrinal and able to build after getting T4, and place Katyushas and Werfers in old Howitzer doctrines. Maybe you will find better way to fix it.
And of course, it won't work with OKW, cos they don't have normal artillery, so, let them have Wuhrframmen in stock. (FOR GOD SAKE, GIVE THEM HUMMELS ALREADY! I REALLY WANT TO PLAY AS OKW, BUT I CAN'T UNTIL THEY WON'T GET ARTY!!!).