The Trade Size alert works off the data that is shown in Last Vol (Last Volume = Last trade size). Keep in mind that if the datafeed is not tick-by-tick, you will NOT be getting every tick, and the sizes in RAW DATA will not necessarily correspond to Time & Sales. You are using TD Ameritrade and do not have it set to get streaming tick data. Therefore, the data MT receives is consolidated. TDA sends 2 or 3 updates per second. Very likely that some rows in raw data represent multiple trades (raw data is showing you the change in volume between the given quote and the previous one). The LAST VOL will also skip updates.
So you are;
1) not seeing every trade, and thus cannot get an alert on every one
2) you are seeing rows in raw data that represent multiple trades and therefore some of them wouldn't have triggered the size alert in the first place.
If you are using the trade size alert, you will want to turn on the tick by tick streaming options. SETTINGS / DATA SOURCES / CONFIGURE ACCOUNTS screen. Click on TD Ameritrade, then check the "Stream Tick Quotes" check box.