There could be a lot of causes for that. The best way to diagnose that issue is to keep the game paused and temporarily cut all your expenditures to nothing one by one and see what happens to your income; oftentimes, there'll be one big thing that's escaped your notice and wreaking havoc on your budget. The big ones for me there are usually:
- Cutting army/navy maintenance. Armies get more expensive over time due to supply consumption rising and population growth leading to army growth. In most of my games, unless my factories are really churning out cash, I can't afford to have a 100% maintained army around the clock. This is doubly so for navies, because they require a steady supply of ships to function and those things are expensive.
- Cutting government employee salaries. Once you've got clergy, bureaucrats, and soldiers/officers up to your desired levels, you can pretty safely cut their salaries down to about 25% to 50% without experiencing a notable number of undesired job changes. Soldier salaries are especially important--if you can't afford to cut army maintenance, then cutting soldier salaries is a decent short-term substitute.
- Streamlining the factories and cutting subsidies for anything that doesn't turn a profit. This was a big one for me at first--industrial subsidies can severely cut into your budget. A good rule of thumb, in my experience, is that any factories that are at least 3/5 staffed or so and still aren't turning a profit by about 1880 or 1890 will probably never work out and can be safely cut.
The reason tariffs and taxing the rich is bad is really important to understand, because both of those are very legitimate strategies if you need to use them or if the circumstances make their negatives irrelevant.
Tariffs are generally bad for any factories that require inputs your country doesn't have enough of. For scarce resources like sulfur, rubber, and the like, you will often need to expand your sphere of influence or colonize to obtain enough for reasonable levels of domestic production. Similarly, your pops need exotic goods like coffee, tea, tobacco, and opium that in many countries aren't readily available; tariffs will, on the whole, put a damper on your peoples' (and especially your working class') drug addictions; by itself, this won't do much, but lack of everyday needs like these can exacerbate tensions by raising militancy or consciousness. Tariffs are thus safe to use if your country or sphere of influence has access to all of the goods you need--mainly what your factories need, though very high tariffs are usually a bad idea. Also note that your intended tariff is different from your actual tariff--without a good administration creating high tax efficiency, you can set the tariffs at 100% and only have like 25% actual tariffs.
Taxing the rich is a problem for a similar reason. Under lassiez-faire and interventionism, your capitalists are your entire source of factories. Especially if you're playing a democracy or a country without access to state capitalist or planned economy policies, not having rich enough capitalists to build factories can be a long-term problem. However, if you have access to or mainly use state capitalism or planned economy, you can tax the rich as much as you like with few repercussions. Similarly, in the early game (when there are not as many capitalists and factories are generally unprofitable,) that slider can be a strong component of your economy.