Especially in the joint alignment at your track connections it is easy to get a jog in the track.
If you can get a camera down looking in line with the rails you can see jogs in the rails at joints and connecting to switches. These are places that might look good just looking but the camera says it all. Where you might have straight rail and go into a curve the rail needs to gently go into that curve and not suddenly have a curve radius. That is like Lionel track using standard sectional track. I have videoed some friends layouts and the camera will be following the rails and that connection joint is not straight It is like a jurk in the rail. My camera has been down mine and it spots lots of places that are not smooth joints although I have not had a lot of problems with them. The camera or video camera can spot places you would not think of. A flex track connection where the connection is in a curve the rails still want to straighten up causing a joint to have a jurk in it. To fix that set your track and cut it to fit as you are installing it. Once fitted allow the rails to straighten up and lightly solder the joint. Then when re flexing the rail it will allow the joint to stay in line through the curve. It will act like one full section flexed for the curve. A joint in a curve using flex track the camera says it all. from Newman Atkinson

Also make sure that ballast is not too high or loose around the tracks, that would cause the couplers or wheels to bounce and disconnect. Be sure that your tracks are clean and clear of anything. A grain of sand stuck to a track rail can cause a derailment of a “N” scale train.