Seaweed is also not great high in iron which our grazing generally already is - and this causes difficulties in magnesium/copper/zinc being absorbed which are required for good hoof growth
I would agree with PFB. I would say your mineral balancing is out, and would suggest getting a forage analysis done. I have had a grass one done and am waiting for the hay to come back.
We are high in iron and manganese, low in calcium, copper and zinc, and I would imagine that the hay would be the same, being from the same area.
Once I have the hay analysis back, I will get a proper plan done to balance the minerals correctly.
I don't think that general supplements are very helpful. Most have iron added, which most of UK is pretty high in anyway, and iron will block as PFB says, magnesium, copper and zinc, so you would need to feed higher levels of those to combat the iron already in your horses daily ration of forage, let alone whatever supplements/hard feed you give.
Edited to add, it is forageplus that I have gone to for analysis, and as Kite Rider says, they are more with it for mineral supplementation than the big companies.