Check where the economizer's wiring harness plugs into the unit's wiring harness. If the unit doesn't "see" that an economizer is connected, then it operates as though there isn't one, and it sounds like that's what yours is doing most of the time. Bad connection at the plug, or the wires to the plug. Could be loose, dirty, corroded,etc....
What type of control do you have. Reliatel or Microtel.
What is the indoor temperature.
What is the COOL set point.
What is return air temperature.
What is OSA Humidity.
What is return Humidity.
What was the position of the economizer.
What was the Supply air temperature.
Do you have comparative enthalpy control or just outdoor dry bulb.
Have you checked the OSA sensor and compared temperature versus resistance and finally voltage drop.
There are many times when the performance of the sensor has degraded and will not respond correctly. Even a simple 3 degrees off will cause an erratic behavior if the OSA conditions are stable.
I just want to make sure that you know you have to wait approximately 3 minutes after powering up the unit before you start stepping through test. The economizer actuator is calibrating during this time, if you don't wait it will skip over economizer and step through mechanical cooling and heat.
Trane units will run the economizer-If the economizer does not satisfy the supply air temperature in about 5-10 minutes, it WILL bring on a stage of cooling to try to satisfy the supply temperature demand - it is 55*F I believe. If you have ROVER software and the Tool, you can view the device and REALLY see whats going on. If your economizer is not equiped with barometric relief or power exhaust- your not getting enough air change through the economizer for it to be able to work. Start up the unit from a warm load, Put a temperature probe in the supply duct and see if you are getting 55* F, if you are not, the mechanical cooling IS going to run. Hope this helps.