Check to make sure that the tip is seated properly. Using a heat pad, apply minimal force to the tip and push it into the handpiece. Do not use excessive force and be careful not to bend the tip.
If that does not resolve the issue, measure the tips at room temperature using a multimeter. Here are the points that need to be measured. If the value that you measured does fall within the tip's respective range, the tip needs to be replaced.
The T39 tips should measure around 5.7Ω ± 10%.
The T50 tips should measure around 8.0Ω ± 10%.