DIAGNOSIS This is almost always a sign of a clogged filter. The 211+ moves a lot of air, but if the filter is saturated with particles, the fan has to fight to pull air through it.
FIX First, check the filter's age. The main particle filter is good for about 6 months under normal use. If it's older than that, it's time for a replacement—no cleaning will fix it. If it's relatively new, take it out and give it a visual inspection. Hold it up to a bright light. Can you see light shining clearly through the white pleated material? If it looks dark and opaque, or if you've been in a very dusty or smoky environment, it may be spent early. Swap in a new filter.
IF THAT DIDN'T WORK With the filter removed, turn the unit on (briefly). You should hear the fan spin up loudly and feel a strong gust of air from the top. If you still feel weak airflow with no filter in place, there's an obstruction in the fan housing or the fan itself is damaged. You'd need to contact support.
HEADS UP Remember, this purifier pulls air from all sides through the filter and pushes it out the top. Don't expect a narrow, focused jet of air like from a fan. It's a broad, diffuse flow. The real test is if it's moving the air in the room, not what you feel with your hand an inch away.