Possible reasons:
1. Problems with the hydraulic system:
*Low transmission fluid level
* Clogged or leaking hydraulic circuit
* Faulty shift solenoids
* Clogged oil filter
2. Mechanical problems:
* Wear of clutches or steel discs
* Malfunctions of the planetary mechanism
* Open power transmission circuit
3. Problems with electronics:
*Transmission control module (TCM) malfunction
*Software bugs
* Faulty sensors or wiring
Diagnostics:
1. Checking the level and condition of the transmission fluid.
2. Scanning trouble codes using a diagnostic scanner.
3. Check the hydraulic circuit for leaks, blockages and correct pressure.
4. Check the mechanical components by disassembling the gearbox (in severe cases).
5. Diagnostics of electronic components using an oscilloscope and other tools.
Solution:
The solution to the problem will depend on the identified cause:
* Topping up or replacing transmission fluid.
* Cleaning or replacing hydraulic components.
* Elimination of leaks.
* Replacement of worn or faulty mechanical components.
* Troubleshooting electronic components or reprogramming TCM.
Note: An automatic transmission is a complex mechanism, and accurate diagnosis and repair require the knowledge and experience of a qualified mechanic. It is not recommended to carry out any repair work yourself.