You said you have 2 different bindings both set to 8 and one of them works fine.
This means either the binding which does not hold you in, is broken or its not set up correctly.
You said it was a STH 14, is it the driver or the Z toe? If its the Z toe, that may be your issue right there, but unlikely. If its the driver, the toe height and wing adjustment may be off. Also the forward pressure could be set incorrectly.
Before you start upping your DIN, go get the binding set up correctly. There is more then just forward pressure to consider on the STH. Remember to take your boots when you get them set up.
If when you have it set correctly and you still pop out, have the spring tension checked. Finally if the tension is good, slower start upping the DIN but only by 0.5 each time.
