Fulfillment of the Contract does not always work?

That's not a bug. Ritual and Fusion Monsters must first be properly summoned before they can be Special Summoned from the Graveyard or banished area.

Fulfillment of the Contract only allows you to special summon a Ritual Monster in the graveyard which was ritual summoned to the field via performing the standard ritual summon. Ritual monsters that was send to graveyard due to other effects such as discarding it or being tributed for another ritual summon are prohibited from Fulfillment of the Contract.

Example:
You ritual summoned PoWD to the field using another PoWD in your HAND as the entire tribute. After successfully summoning PoWD to the field, you will have another PoWD in your graveyard. However, you CANNOT activate Fulfillment of the Contract to special summon the PoWD to the field since it was not being ritual summoned to the field before it was in graveyard.

Hope this help clarifies your confusion. Sorry for my bad english too.