'nibble' type sucking

DD is 1mth old. Today, all she's done is snack. Not properly feeding. She had a proper suck this morning around 8. She just keeps falling asleep. She'll wake up about every 2hrs, maybe have look around, watch her brother and be content to just sit there... But its a few sucks, then keeps the nipple in her mouth and every so often 'nibbles'. She'll spit the nipple out on her own. If I tickle under her chin she just nibbles that bit stronger...

What can I do? From about 430pm she normally becomes a vampire and just latches on and I can't do anything. It's exhausting.

Is she getting what she needs today?

It's frustrating hey. You can't make them eat, even though it can make life difficult.
Is she otherwise well? Not snotty or anything?
In a baby that little I would still be offering every couple of hours and then resigning myself to the fact that she'll be overtired and needing extra feeds/cuddles towards the end of the day.
I'd reckon she will make up for it later on. She may have some really good solid feeds and sleep better for it at night... maybe :)
To sum up, LOL! I think you are doing great. Keep offering. She knows what she needs. I'd only be concerned if she doesn't seem well, but even then there's not much more you can do than keep offering, if maybe a little more regularly.

Jack used to do that to me. I don't know if I had any success on doing anything that made it stop/change. I was told to take him off & attach him again until he drank properly. But I gave up in the end. It wasn't worth the stress, so I just accepted it.

I call it flutter feeding :) Comfort with a bit of milk on the side.
You can tell best how much she's getting from her nappies - lots of wet ones? If everything looks right (nappies, weight gains, etc) then you just have to trust that she's getting what she needs.

You can also do some exercises before feeds to help with muscle tone around her mouth. Tummy time will also help develop and strengthen her mouth so she can suckle nice and strong.

If she's alert, it's good! And it sounds like she is alert, so well done!