I buy low-priced and penny stocks a lot. I run into this situation constantly (even on higher-priced stocks.) I bid, say, $.10 (with no qualifiers) on 1000 shares on a stock currently selling for $.15. It will creep lower toward my bid (sometimes over a 6-month time-frame) and suddenly one day it sells—to someone else—for $.07, 3-cents below my bid. The next day it will pop back up to $.12 or $.15 and I get filled on no shares at my bid price. Is this not a "bidding process?" If I'm bidding higher, why don't I get filled first? Why does someone else get to buy the stock I want cheaper than my bid price?
It definitely depends on the exchange you are trading on. I'm not familiar with Scottrade, but a standard practice is to fulfill limit orders in the order they are placed.
Most of the time, you wouldn't see stocks trade significantly under your bid price, but since penny stocks are very volatile, it's more likely their price could drop quickly past your bid and then return above it while only fulfilling a portion of the orders placed.
1. Penny stock priced at $0.12
2. Others place limit orders to buy at $0.10
3. You place limit order to buy at $0.10
4. Stock price drops to $0.07 and some orders are filled (anything $0.07 or higher) based on a first-come first-served basis
5. Due to the increase in purchases of the penny stock, the price rises above $0.10 before your order is filled
***EDIT*** - Adding additional clarification from comment section.
A second example
If the price drops from $0.12 to $0.07, then orders for all prices from $0.07 and above will start to be filled from the oldest order first. That might mean that the oldest order was a limit buy order for 100 shares at $0.09, and since that is above the current ask price, it will be filled first. The next order might be for 800 shares at $0.07. It's possible for a subset of these to be filled (let's say 400) before the share's price increases from the increased demand. Then, if the price goes above $0.10, your bid will not be filled during that time.