If client is set to stop or is already ended and you don't know why, you can try open up the log for the subscription and try to see, if the log tells you why.
In the case under here the log for the subscription says that the client cancelled itself.
This is possible if either:
A client portal is a part of your setup (where clients can change payment details or cancel subscription
Are paying through inapp purchase (Apple or Google) and has cancelled their payments this way
To find the log open up the client and do the following
Right-click on a subscription
Choose Advanced settings
Select History at the left
