The main reason for doing IP warmup is to gradually increase your volume, without hitting the daily capping limits for ISPs. I.e. you don't want to hit Hotmail with 25.000 email on the first day of sendouts. If your volumes are within 20k for all your subscribers, I will not see IP warmup as being needed.
When you talk about small domains - how many addresses are within each of them for every send? If this is within few hundreds, indeed IP warmup is not the issue. Usually those small ISPs have inferior spam algorithms, which are not as intelligent as e.g. Gmail. There is nothing else for you to do, than continuing sending to them. What you might be seeing, however, is your IP being blacklisted - something you can verify in this tool.
I will also recommend you keep an eye on your SenderScore, giving you an indication of your overall reputation. The timeline can also provide you insights into any specific sends causing drops in reputation.