hmm..by the sounds of it(clumpy black ash that is hard to flick) seems like it might not be a weed problem but a packing the joint prob. Try rolling a bit more loose with it...and make sure its ground super fine...and see if that takes away the black clump of doom
I know if I roll super wet sticky weed and it's packed too tight/not ground enough then i get that black clump.
If its not that... then yeah, that sucks...