Dropdown list on multiple true values?

Hi, I'm having some trouble creating a dropdown list that catches all instances in which a certain value is true.

In the sheet "Contractors and contracts" I have a list of contractors, their contract numbers, short description value and concepts. That sheet is working fine. In the BILLS sheet is where I'm having some trouble. What I want to do is that when somebody chooses the Contractor it pops a list of contracts for that contractor, and then from there it pops a list of concepts for that contract. The list of concepts is working as intended, the problem is the contract is simply not working at all. Could anybody shed a light to my issue? I'm sure it is something very simple that I'm missing but I just can't find it.

I actually read that post to do the second drop down list... The problem is I have multiple contracts with the same contractor, and it will just find the first one rather than giving me every instance in which that contract is true

The problem with the way they are doing it is that they create a few support tables to get what they need. The way we're (and by we I mean me) are currently doing involves copying the same information in multiple places, creating support tables sort of defeats the purpose. I was looking for a simple easy to understand way of just adding a new contractor and a new contract all in one table (hopefully keeping the format I already established)