I do not think it is the other cities fault at all. You take the same path I tend to do, I do not have that issue.
One thing I did find that helps me maintain steady growth thru out the whole game is… On your small map, do not open a factory until you have all 7 cities with their own intercity bus routes. Then open up the 6 city to city connections to your hub city. Just an FYI here, I do not even have a factory open until 1885- 1890. But doing this, I can still hit 1000-1100 by 1930,
Wait for a little growth, then open up your middle most factory, feed all 7 cities with that. It will grow kind of slow at this point. But it will expand and feed all 7 cities for quite a while. When that plant hits 380ish production, find a north or south factory, open it and feed one of city’s near it. After it expands, feed a 2nd city.
When your main factory gets back up to 370-380ish, open your 3rd factory, with one city being fed, when it expands feed 2nd city.
This gives me constant growth thru out the game,I tend to hit 1000-1100 in hub by 1930. 2-3 other cities will be at 550-600. The rest either high 400’s or 300’s.
What I noticed for me was, if I open a factory early in game, 1850-1855ish, that the factory expands rapidly. Then my cities stagnate, even with 3 factories running. Not sure if it is in my head, but after doing this, I have not had stagnation in my games.
Also note, that doing to much cosmetic road work in hub city can and will hurt your growth. So, if you like a ring around the city, make it huge. Also, the cities will grow towards your bus stations, use that to your advantage.