The dust door on the ejection port of one of my rifles will not catch to shut. The detent seems to be pretty soft. What do I need to replace?

Is the rifle new? The little notch in the upper where the detent engages could be worn.

not brand new but new. Seems the detent is a softer spring than my other rifles.

Link below has an ejection port cover assembly and other parts.  There's probably a ton of places that sell them actually.

This is assuming the problem is the door mechanism itself, and not erosion of the metal where the detent pin makes contact on the upper, or something else out of spec of course.

Me, I don't even know if I would worry about it for as often as I close my dust covers (which is almost never), but if you are in the field a lot with your AR then I can see how it would be a problem.

What brand AR is this?

Dust Cover Parts

I dont care much but Im selling it to a guy and want to make sure it works for him in case he uses it.

Its a CMT upper

Get a new cover and a new snap ring (retaining clip).  That small ring is very easy to mangle when disassembling, IMO, it's just easier to replace (buy two, just in case) .  Midway and Brownells should have the parts.

Pop the clip off, slide the hinge pin out (being careful not to launch the spring), remove old ejection port cover.  Reinstall is the reverse, you might have to mess with the spring to get it to seat correctly.

so nothing I can do with the existing one that will make it work?

You could try and stretch the spring.

Just slam the damn door man, just sayin' man

I have had this happen on a few occasions and i just removed a bit of the tip on the detent with the dremel on low speed....it never took me very much at all, just a smidge so go slow should you try this.