whos at fault, me or my landlord?

According to my landlords they never received my check (i pay through wells fargo bill pay), but according to wells fargo they sent the check and it arrived on the 1st. when i sent an inquiry to wells fargo they claimed that no mistake was made on their part, according to my bank account every things been sent and accounted for, but the checks not been cashed. now my landlords are asking that i pay rent plus 70$ in late fees. so whose at fault here?