M1 will make a best effort attempt to execute and fill all buy/sell orders. M1's automated trading includes all buy/sell orders during the first trade window after submission. You can always see your pending calculated trades in your account's 'Activity' tab. There are a few circumstances where buy/sell orders will not be executed to match your explicit instruction.
When will buy/sell orders be fulfilled?
All trade orders are included in the next trade window.
Example: A user inputs a buy order on Saturday, this buy order will be included during Monday morning's trade window. More info on trade windows.
What can cause a buy/sell order not to be fulfilled completely?
Lack of cash available to complete buys
If you do not have enough cash available to trade for all buys orders, M1 will have to pro-rate the orders to match the available cash
Example: A user submits a $100 buy order for Stock A and a $100 buy order for Stock B and has $100 in available cash. M1 will prorate these orders to $50 for Stock A and $50 for Stock B
You can always see your available cash on the buy order submission screen. Cash available to trade includes all cash in your account, incoming deposits, and pending sell orders
Fractional Share Limitations
When you trade fractional shares you can buy up to 1/10,000 of a share. While this allows us to trade in small increments, there is a limitation to precision.
Example: If Stock A is selling for $900 per share we can purchase Stock A in $.09 increments because 1/10,000 of a share of Stock A is $.09. If a user has a $100 buy order of for Stock A, the closest we can get to $100 using a $.09 increment is $99.99.
Sell orders not receiving enough to make buy orders
When you submit buy and sell orders during the same trade window, our system will sell first and then buy using the received funds. If the received funds from the sell is less than expected, the buy orders will be prorated to the funds available