Most likely explanation is pollination issues - lack of or insufficient. Tomatillos are noted for pollination issues - one reason why many plant several to increase the odds of successful complete pollination.

But there can also be other causes, many of the same that affect tomatoes - plant stress, inconsistent soil moisture levels, too small a container if one is used, excess nitrogen fertilizer, high temps and humidity, etc. Check out the FAQ here on the condition called "blossom drop".

Dave