The issue is the vacancy. Empty houses are more prone to every type of loss, but in particular insurance carriers have been dealing with "human termite" claims given the spike in metal prices. It's not uncommon to see $20,000 claims from damage done by thieves who got $500 worth of copper.
Do you flip the houses in your own name, or do you have an LLC? Find a good independent insurance agent* and talk to them about how you might structure the policies to best fit your situation. One thing that might work, depending on ownership and everything, would be to get a Business Owners Policy (BOP, pronounced "bop" rather than bee-oh-pea) that has a "Newly acquired or constructed property" clause, if your bank will accept that.
*i.e., not your friend's brother-in-law just because he's been cold-calling you for years. Interview them, give them your info if you feel comfortable after the first meeting and see what they can do for you. If you're small, you'll get the best service from a small agency, and it doesn't sound like your risk is unique enough to need the services of someone larger.