Not easily. Unfortunately some publishers lock down their fonts in such a way that (as in your case) you can only adjust the size between too small and too large, or you can't increase the size at all, or you can't switch between serif and sans-serif, which makes the result far less user-friendly. There was a huge uproar on the day JK Rowling released her last book because it was basically unreadably small, on either Nook or Kindle. They released an update, but for less famous authors there's no outcry so the publishers won't fix it.
I've also seen them lock up the margins, which can be very annoying. If you put 1-inch margins (and I have seen it) on a 5"x7" screen, you now have a 3"x5" effective page size, and more than half the screen is white space.
If you really want to go to a lot of work, you can use the Calibre product to strip the encryption off the file, resave as a rich-text file, change to a standard font, and reconvert to an EPUB. I've done this a few times for my visually-impaired mother.
If it's intermittent (not on every book), I would think this is a publisher issue, rather than a device issue. If you notice this with one particular publisher more than others, it might be a good idea to drop them a line and indicate the issues you are having.