I know that a good alttitude to intersept the glide slope is 2800ft. This is 2800ft above the alltitude of the airport/runway right? so if the airport is 5000ft like in Denver you should be at 7800ft right? Different airports don't have different intercept hights do they? Recently my 737 has faild to lock on the glide slope and I don't know why? I think I have my speed and alt right. 160knts
Yes, all altitudes to do with the ILS are AGL i.e. above ground level, not QNH (i.e. AMSL). Interception heights might be different in the real world for different airports, depending on the approach. For example, London City airport (EGLC) has an glideslope of 6 degrees, so interception height will be a lot higher - but in general, 2800ft AGL will suffice.
You should lock on if you have your GPS/NAV switch to NAV with the correct NAV frequency with a speed that you have mentioned. If you aren't locking on, them I'm not sure what the problem could be.