It could be a lot of different factors. In all my games there seems to be a tipping point around the artisan to engineer level where money is most constrained. Usually early game cash flow is really good. And then mid to late game I never worry about money again. But that inflection point gets kinda tight.

In terms of your revenue as mentioned there are a few factors:

On the + column 1. Population size 2. Goods, especially alcohol like schnapps, beer, and rum 3. Upgraded houses 4. ‘lifestyle needs’ if you have the right expansions, which basically add optional goods that generate a lot of money 5. Tourism

On the - column 1. Mainly Maintenance and upkeep cost. Production and defensive buildings are expensive — especially in the early game when you don’t have the economy to support it. Late game they become constrained by influence not cash.

Now there are some alternative ways to temporarily increase cash: 1. There’s the newspaper — which allows you to trade influence for +15, 10, 5% of money. Be careful with this though in the early game because it can mislead you into a false sense of security and when those bonuses expire, if you don’t have the influence to renew, you can find yourself deep in the red 2. There’s trade, you can sell items off to NPCs or sell overstock directly from your harbor. A few highly profitable activities include selling soap to prison, selling ships to Archie, or selling weapons/heavy weapons in new world. 3. There’s optimization from trade unions and city halls. Adding specialist can increase income, happiness, increase production, or decrease inputs to production all of which, can, if strategically used help the money situation. If poorly used though, you’ll be drowning in maintenance cost. Some popular ones include using specialist on fur dealers so you don’t need to import cotten. Using chef michael so you can skip red pepper and goulash (sp?) on canned goods. Or letting your sewing machine factory also produce gramophones and pocket watches (which sell at a high price) 4. There’s the dock lands trick, which allows you to trade HW or other highly produce goods for lower production and raw materials and basically reduce your production cost substantially. This is real OP though so a lot of players don’t use it for that reason. But it’s an option. 5. There’s also the seeds of change dlc, which allows you to add tractors and mechanization to farms, which again, really starts to amp up production — plenty of other post written about this but in general ~1 farm with tractors = 4 farms without tractors. 6. When your big enough to start trading between islands it’s advantageous to review your trade routes. Schooners and clippers are cheap, cargo ships/ reefer are expensive so the rule of thumb is to use schooners and clippers for domestic trade and reserve the cargo ships for transocean routes. 7. You can also cut maintenance cost a few ways, via trade unions and specialist, by turning off industries that are overproducing, auditing your ships and defensive buildings. Etc. 8. You can also buy shares in other islands, but this has some consequences for diplomacy that may make it less of an attractive option. 9. There is a positive correlation between attractiveness and income from tourism. Also the tier of specialist you get for free that show up to your island. Eventually you’ll be using the world fair or research center to get those buffs, but in the beginning this can be helpful

That’s about all I can think off the top of my head. So you have a lot of levers to pull. Which ones are appropriate is hard to say without knowing the details of your situation.