CABIN ATTENDANT REPORTS THAT O2 MASKS FAILED TO DEPLOY IN A TIMELY FASHION DURING LOSS OF PRESSURIZATION INCIDENT ON B757.

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APPROX 20 MINS AFTER TKOF; THE CAPT MADE ANNOUNCEMENT FOR THE PAX AND CREW TO TAKE (USE) THEIR OXYGEN MASKS. I LOOKED INTO THE CABIN AND THE OXYGEN MASKS HAD NOT DROPPED. I CALLED THE PLTS (WITHOUT THEM ACKNOWLEDGING ME) AND SAID 'THE MASKS DID NOT DROP!' BUT FELT I WASN'T BEING HEARD. THE #4 FLT ATTENDANT AND I PUT ON OUR SUPPLEMENTAL OXYGEN UNITS; AND PROCEEDED TO HELP THE PAX. BY THE TIME I GOT TO MY FIRST PAX; THEIR OXYGEN MASKS HAD FINALLY DROPPED; AND THEY WERE TRYING TO PUT THEM ON. I HELPED ALL OF THE PAX WITH THEIR MASKS; WITH THE HELP OF THE OTHER FLT ATTENDANTS. WHEN ALL OF THE PAX WERE CHKED; WE NOTICED 'A SLIGHT FOG' OF SMOKE IN THE TOP 1/3 OF THE CABIN. I THOUGHT IT WAS FROM THE ACTIVATION OF THE OXYGEN UNITS. WE (FLT ATTENDANTS) MADE SURE EVERYONE WAS ALL RIGHT; AND THEN THE AIRPLANE LANDED BACK IN ZZZZ. IT WAS A VERY STRESSFUL SITUATION WHEN THE PLT MADE HIS ANNOUNCEMENT TO TAKE OXYGEN; AND THE OXYGEN MASKS DID NOT DROP. PAX WERE TRYING TO PRY THE CEILING UNITS OPEN TO OBTAIN THEIR MASKS. NO ONE SUFFERED FROM HYPOXIA. APPARENTLY THE AIRPLANE'S L PACK AND BLEED WERE ON PLACARD AND SUBSEQUENTLY THE R PRSOV FAILED; THUS NO AIR COMING IN FROM THE ENGS.