@raghav - I responded very well to P-5-P also. Nutreval testing showed I had a severe deficiency. At one time I had to take 150 mg. a day, and now take 100 mg. (50 each with breakfast and lunch - at night it causes insomnia for me).
I don't know the answer to your question and why it stopped helping you. Just throwing this out there, but you might be better off with a different B complex. Jarrow makes a good one called B-Right optimized B complex. It has folate instead of folic acid and methylcobalamin instead of cyanocobalamin.
Would it be possible for you to get Nutreval testing done? It could help pinpoint your deficiencies and what you don't need.
Also, maybe you need more P-5-P. You might try increasing your dose and see if it helps.
I did need more thiamine - I now take 100 mg. thiamine and 150 mg. benfotiamine a day and that boosted my energy as well, plus I take the Jarrow B Right, plus the P-5-P, plus 5000 mcg. liquid methylcobalamin 2 x a day, plus 1600 mcg. methylfolate.
There may be cofactors needed for the P-5-P but am afraid I don't know what they are.