Tekkit crashing with additional mods.

[MisterMooses]

There are a couple extra mods I'd like to add to my Tekkit version, such as Tinkers Construct, Shaders, etc. but when I put any additional mod into the Tekkit mods folder, it won't launch. It goes the file verification process, the window opens for a second, and then closes again. I have done this before, with both Hexxit and Voltz, making them custom modpacks with mods from other packs and mods I downloaded separately, so I know it should work. Was wondering if anyone has any idea what I might be doing wrong or how I can successfully add more mods to the pack.

[TOSU1]

Are you using the correct 1.6.4 versions of the mods?