I think you may have answered it when you said you have a modified square wave inverter
. Try plugging in to a true sine wave voltage source and see if it works. Some things just don't work as well or not at all with modified square waves. Some things in fact can be destroyed by modified square waves...I have destroyed a couple power adapters with one.
You may also not be getting the correct RMS voltage that is required.
I say modified square wave because that is what it is. You would never want to modify a sine wave only to wreck a perfectly good sine wave.