Resolution on same monitors not the same

So I have this litle issue. I have two identical monitors hooked up to my mac mini, one through HDMI to vga conversion and one through thunderbold to vga conversion. The monitors are exactly the same model and brand. The only issue is that some times one of the monitors resolution is awful. It doesn't happen in the middle of me working, but more or less if I disconnect it and reconnect it or something along those lines. When I try to manually set the resolution, it doesn't display my optimum resolution like then other monitor does. Anyone have an issue like this before and have a resolution? I feel like I am just over looking a setting, but I am not 100% sure. Any help is appreciated. Thanks community!