Please check from the below three aspects:
1. Current input to split;
Please power up the Split via USB by a phone charger (5V 1-3A) to see if it works properly. If it does, it indicates the problem is caused by insufficient power. Please then try to change your power setup to PDB BEC /FC.
2. The connection between the 3pin JST cable and Split's main board. Please check and make sure it is stable and try to fix it with tape if there is looseness.
3. You may also try another card, please refer to Which type of microSD card is fit for RunCam Split?.