Problems with dll are often resulting with dependencies to other dlls, This is known as "dll hell". So welcome ;-)
I would recommand using the tool
dependency walker. This great tool helped me in a lot of crazy scenarios and solved my headaches. Pay attention to x32 and x64 flavor of this tools (and your dlls)
profi-tip: using static linking as often as possible and check that the dlls are in the search path or in the correct directory which also should be starting directory of the process.
The mantra is: user dlls are in the installation-directory of the app, but system or third party dlls ware installed with the installation software of the vendor.