Over all, that may be true about Polish. Are there any specific areas of their growth that are of concern? And it would help to know how old these chicks are. Growth concerns vary drastically from those during the first few weeks and those as a chicken approaches maturity as they get close to four months.
Where are you brooding them? What temperature? Have you been gradually decreasing the heat?
Feather growth may speed up or slow down according to the hotness or coolness of the environment the chicks are in.
Some chicks brooded indoors under a heat lamp may be much slower in feather development, especially if the heat is kept constant over several weeks instead of being reduced. Conversely, chicks brooded in a cool environment will completely feather out in as little as three weeks.
This may or may not be an issue with your chicks, but just thought I'd bring it up. We have no idea what your geographical climate is, which may contribute to this issue, or what your brooding setup is. We're just trying to fill in the blanks by guessing.
Diet also plays a role in feather development. What are you feeding your chicks?