Firstly there has been doxycycline resistant cases. Secondly is there a compliance issue or is the doxycycline taken for too short a duration ( typically a minimum of 3 months ).
If not then one can consider switching to isotretinoin like Roaccutane either alone or in combination with minocycline.
For recurrent cases one can consider Agnes RF which destroys the sebaceous glands and thus eliminates the root cause of the problem.
Hope this helps.