Its not an issue of an absolute monarchy (or a Prussian Consitutional monarchy) not being as omnipotent as the ideal holds. The problem is that for countries that start with little or no voting franchise, the politics tab of the game might as well not exist. Having played through as Two Sicilies and Japan, I passed reforms through the upper house a grand total of two times, with two other reforms occuring through events. The same political party was in power for the entire game.
I'm not sure what you're doing, but I just ran a quick 2Sic test and by 1940 my UH is 35% liberal and I've just passed my first reform: Landed voting. UH is appointed, and there's only 50k aristos in 2Sics, so once nice liberal capis start showing up they'll quickly make the UH even more liberal. Middle class is hugely liberal too, so I can aim for wealth voting + based on population UH and have ~60% liberals until Socialists show up.
Been playing Prussia recently, who also start with no voting, in a similar way and 30 years in I have have hugely reformed my country:
I stayed at Wealth voting until Socialism appeared as my poor pops were pretty conservative, and stayed on underground parties as I wanted to keep reactionaries in power, could have had all political reforms if desired though.