Payment on hold

I have just sold a iPhone and it says payment on hold. It did this the first time I sold somthing but I’ve sold other items since and it didn’t do it so why has it done it again??
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New/occasional/returning sellers are liable to a 21 day hold on payments.   Login PayPal to check the payment status and messages there (eg. pending, safe to send to the given address etc.)   Entering tracking details showing sent/delivery to the address given in PayPal is said to release funds in around 7 days.  

Without tracking, you will need to wait for payment release.    Feedback is not taken into consideration in releasing the payment.
https://www.paypal.com/uk/webapps/mpp/fundsavailability