Does anyone else have problems doing this? I can barely ever get them to ring out at a reasonable level on the e/B/G. And doing an open hammer-on (I don't know if this is the correct term for it, I'm talking about hitting the open string then hammering a fret down) is pretty much impossible to even hear, honestly. My hammer-ons on D/A/E are all the same noise as a strum, so I don't really think it's finger strength, but maybe it is?
Make sure you're hitting right by the frets- it makes a difference.

Could be finger strength, yes, but (as you can do it on the E A and D strings) you could also be using too much force (which would deaden the note).

What you need for this kind of legato playing is a short, sharp attack (like a hammer). Also, the movement has to come entirely from your fingers.