How are you creating your matrix? Are you doing it within and audio console? Are you doing it within software? Or, are you using an actual MS decoder?
If you are using an audio console, the Mid microphone be it an Omni or cardioid microphone, must go into a single input on your console, pan to center. Your figure 8 microphone must be split and fed into 2 separate inputs. 1 panned left in phase in the second input, panned right, phase inverted. The levels on the figure 8 microphone must be balanced so that if you pan both to center, it will cancel completely. Since it is the same signal with one channel out of phase to the other.
Next, take the figure 8 inputs and Pan them both back to left and right. Now with the figure 8 levels matched together, slowly bring up your Middle microphone panned to center and voilą! You can adjust the stereo width by slightly increasing or decreasing your center Middle microphone. If you remove the Middle microphone completely, you will hear your figure 8 microphone completely out of phase through both of your speakers. This is what makes this matrix work. Your Middle microphone should not disappear! But in mono that figure 8 microphone does disappear if the levels are balanced properly.
I know that MS should work regardless of Omni or cardioid microphone although the true definition of MS generally indicates a middle cardioid microphone.
If you are using software and losing the Middle microphone, reverse the channel's as what you're describing indicates that your Middle and figure 8 microphone are reversed.
I love MS miking!
MS Remy Ann David
Why do you think they call me "MS Remy Ann David"?
It's not because I'm beautiful.