Wyckoff trading method, page-1489

Ummm, in a perfect world, yes, a reduction in volume each time price makes a minor challenge to the breakout line or creek would be expected.
Usually though, the accumulation zone mops up all or most of the supply within its borders, but not all potential sellers will allow themselves to be shaken out and sell for a loss. They will wait above the breakout line (creek), very close to their breakeven level, and will sell into a markup. And it is this supply which can be inconsistent, changing with the personal circumstances of the individual holders.

cheers