Forecast split into weeks has problem

Forecast is loaded in months. When choosing weekly bucket to display in the planning book, for some products, forecast is correctly distributed equally in all 4 weeks. But for some products, whole monthly forecast is put into one (last) weekly bucket for that month. I do not know why this is happening. Since this is for some products, I assume it is due to some master data setting. What fields should I verify? Or in general also what configs or settings should I check and now, in order to solve this?