Tissue remedies do work, the question is did you took the right one. You can't prescribe a remedy only based on one symptom. Your fatigue must be accompanied by other symptoms. Usually tissue remedies are prescribed in combinations for optimal effect.
Symptoms calling for Kali phos are ash-coloured face, greyish skin, as if not washed; the temples are concave; the look of the face is tired.
Usually 3 weeks are enough time for feeling the positive effect; if not, make some research to find the right salt for you.