my heavy bag is filled with shredded clothes and i put 3 bags of sand right at the top to make it alittle heavier but i get some really hard spots and i dont really understand how it happens since its filled with clothes, i usually take the bag off and beat it or leave it upside down but it takes a week or 2 to start getting hard spots again, what should i do?

i thought maybe i could ge the sand together in a big dustbin bag and rolle it up into a ball and put it right at the bottom of the bag?