Melanie,
If you want to email me a quick video of you playing I can take a look at your technique.
It could be that you’re not holding down the second note with enough pressure.
A lot of times though, I’ve found students actually aren’t giving the first note in the hammer-on/pull-off enough value.
You need to hear the first note clearly so you have somewhere to either hammer-on or pull-off too.
I’d try giving the first note a little more value and then yes, you need to keep the pressure down on your other finger.
But it shouldn’t take too much strength or pressure.
Feel free to send a video though and I can offer some specific feedback.
– Mike