Frenzel and Valsalva are in fact rather similar methods, with the difference that at Valsalva you push the air from lungs, with the help of diaphragm, while at Frenzel the epiglottis is closed (hence lungs isolated), and you push the air by reducing the volume of air in the throat.
It means that if you cannot equalize with Valsalva, it is likely you'll have the same problems with Frenzel too. The problems is not in the method, but probably in the way you do it. Only rarely the cause is really physiologic. Sometimes, at allergies or infections the sinus may be swollen. At beginners the more common cause is that they simply wait till it hurts before they try to equalize. That's too late - when there is a big pressure difference, the e-tubes collapse and you won't manage to push the air through them. You have to start early, in fact already on the surface, or just after the immersion, repeating the manoeuvre very frequently as you descend.
Basically, if you can equalize (resp. make your ears pop) on surface (dry), you have no physiological problem, and will be able to equalize in water. If you cannot, you either waited too long, or are stressed - try relaxing or sinking feet first. Sometimes the problem is that people simply do not blow hard enough. However, in no way you should try equalizing by force - under water you do not need more force than on the air. If you can't equalize, surface, clear your sinus and try again. Or just ascend a bit and try equalizing.