The p999 is the G2x which is the T-mobile version as im aware of.
the P990 is the O2X which the the english Lg version
the reason why it isnt working is because the P999 has different partitions and such.
SmartFlash bassicly puts your phone back to stock 2.2.2 froyo without root or anything, its as if you just got it out the box
ill try and find a link for you and get back to you.
. i couldnt find a smart flash for the P999 for some reason :S but this might help