I´m using exactly the same engine to get started, and I had exactly the same problem. I have fixed it by returning a different value for each kind of object you can collide. (For example, objSolid returns 1 and objPlatform returns 2).
Just with that change, in the same part of the code you posted, it won´t work (objSolid collisions will work, but objPlatform collisions won´t work at all!), because it seems only 1 is considered "true" for GM. You have to go to scrPlayerYMovement, scrPlayerXMovement, scrPlayerPerformXMovement and scrPlayerPerformYMovement and change some questions to the parts of the code that are related to move the player out of a collision. In some questions, you will want to ask for 1, in others, for 1 and 2.
After some tries and not much changes, you can make objPlatform collisions to work better. You should make a 16 height collision too for these ones, to minimize trouble.