Some worms cannot be seen with the naked eye. Here is some information on common worms on the page are additional links that describe common symptoms for each type of worm listed.
Two wormers that have been recommended on the BYC forum have been Valbazen or Safeguard equine wormer (liquid or paste) with the main ingredient Fenbendazole. Both will treat most worms except tapeworms. Wazine treats roundworms only so go with one of the other two mentioned.
The other possibility could be coccidiosis and that is only treated with Amprolium under the brand name Corid. Here is some information on that from the BYC Learning Center