I noticed a severe drop off in my PoH numbers since about 2 weeks ago. Sorry about the links, not able to post them yet.
Here my PoH is healing for 24.8k and critting over 40k. (from 12/11).
worldoflogs. com /reports/z5j8onvs8fz2sfqo/details/3/?enc=bosses
The last 2 raids the numbers have dropped to 17k with crits from 21k to 30k
worldoflogs. com /reports/4q3d4mpbk3gvgt1w/details/9/?enc=bosses (12/18)
worldoflogs. com /reports/4q3d4mpbk3gvgt1w/details/9/?enc=bosses (12/20)
If anything, my gear is better than it was before. Looking back further I see better PoH number there too. Checking buffs, I don't see anything missing. Here is my armory:
us. battle. net /wow/en/character/area-52/Tandrelyn/advanced
The numbers you see on your page on WoL are effective healing values without including overhealing (even if the overhealing value in percentage suggests otherwise). In your more recent raids, your PoH had more overhealing and therefore gives the impression that it's healing less (like, if it hits for 100k but 90% of that goes to overhealing, it'll show as if it only healed 10k).
If you want to compare raw values, I would suggest checking the "Log query" and checking your numbers for Prayer of Healing there.