Keep trying, For my duplex's i ended up using a local mortgage co and received a 30 year fix at 3.375 with .66 points. And yes, closing costs are high for duplex's right now, so make sure its worth it. With your current rate, at 5.375, that's $140 a month in savings per 100k borrowed. If you are planning on holding these for long time, I'd make it happen. Google mortgage companies and start calling the ones online.
Here are 2 online companies i've used in the past.
First Bank - You can only use these guys if you have 4 or less properties but they were by far the best mortgage co ive used for rates,fees and ease.
Security National - this is the one i use for when a less than conventional mortgage is needed (read spendy company)
FWIW My single family rentals were much cheaper to refi, there were no point and the rate was 3.25.