It's possible for a defective script to do this. You can test in that channel's editbox with:
//!msg $chan test
You can also check your ignore list to see if you've put yourself on ignore. That can prevent you from seeing your own /notice but shouldn't block /msg
(small letter L)
If your text appears when you highlight the entire line with your mouse, then somehow you've set your text to be the same color as the background.
mIRC is supposed to block the effect of someone setting foreground and background to the same color, though if 2 color numbers are set to a shade that's very similar, it allows that. For example, the default 2=blue is hard to see as text against a black background.