Make sure
1. the hearth and spindle are dry,
2. the spindle is straight as possible.
3. your technique is locked in.
by 3, I mean keep the foot securing the hearth as close to the spindle as possible, and hook the arm that supports the bearing block around the knee.
this will stop the spindle from wobbling.
Keep your weight above the spindle.
Move the bow slowly and smoothly to start, then speed up.
when you get smoke, speed up a little. when the smoke starts to twist around the spindle, do 30 strokes as fast as you can.
lift out the spindle and sit back.
You should have an ember by now, but don't rush things.
The ember will last for a couple of minutes. Get your breath back.
You don't want a drop of sweat to fall from your brow and land on the coal (this has happened to me, it's not funny.)
Once you've composed yourself, lift the hearth off the ember, and introduce it to your tinder bundle.
Hope this helps.