Yeast starter questions

Hi, I've made my first yeast starter 24 hours ago. I use Whitelabs Belgian Wit yeast. I've made a 600 ml wort at 1.040 with pale DME. There is a slow yeast activity (few movements in the wort) but no bubbles in the airlock. I know it's pretty soon to worry about, but it's the first time I see fermentation start and then kind of "stall". Nothing seems to increase. So my questions are:
-Is it a problem not having a full liter starter (about 600 ml in my case).
-Is it slow because of the quality of the wort? the yeast?
- Should I pitch even if the fermentation looks very slow? (I plan to pitch in 2 days)

Thanks for your help!

1) No, but there's no reason not to go bigger (unless you don't have the capacity?). FWIW, I do 1/2 gallon starters in a growler...

2) It might not be "slow" at all. you won't see airlock bubbling from a starter like you will at high krausen in a full batch, just due to the volumes/populations involved. In fact, many people don't use airlocks on starters, as having a steady supply of air/oxygen helps the yeast reproduce, rather than going into fermenting mode.

3) Yes, if you're seeing any activity at all (you could take a hydrometer reading to verify that the yeast is eating sugar, but probably not necessary). If you have two days (and the capacity), I'd step the starter up to 2L.

1 liter would be better, but 600 ml would be much better than nothing.
It's slow because it is a starter. The first day or so, the yeasts will be reproducing, and producing very little (if any) CO2. Just more yeasts. Even when the yeasts start fermenting, you will not see much activity in the starter because the starter is so small. (I'm not talking about 600 ml to 1 liter, just the size of a starter in general.)
If you want to, you could step up the starter tomorrow by adding another 400 ml of 1.040 wort.
By pitching time, you should see a considerable increase in the volume of yeast, although you probably won't see any signs of vigorous fermentation.
Did you oxygenate the starter before pitching? This really helps the yeasts to bud.
You should remove the airlock from the starter, and replace it with some sanitized foil or an inverted sanitized jam jar. This allows air into the starter without allowing anything nasty to drop into it. If you don't have a stir plate, you should also give it a quick swirl as often as possible. This keeps the yeast in suspension, and helps to introduce O2.
And most importantly, RDWHAHB

-a.