You could have a frozen pipe (actually, it would take two in this case to completely isolate the upstairs bleeders.) You might not. Don't know the layout of your pipes...

Circulation pump does not need to be running for bleeding. System pressure alone should suffice for bleeding. "Hissing" when you bled before sounds like trapped air. When you start to get water mixed in, close the bleeder, wait a few minutes and re-open. You should eventually get just water, no hiss, no bubbles.

I'm going to suspect that you have low pressure in the system, so air is not being forced out the top bleeders by water pressure in the system. You should have a pressure gauge (often on the same face as a temperature gauge) on the boiler - what does that read? Normally it should be 20-30 PSI. You may need to add water.

If you can't sort it out yourself, call for professional help sooner rather than later, as letting the system freeze can result in considerable expense.

You could have a frozen pipe (actually, it would take two in this case to completely isolate the upstairs bleeders.) You might not. Don't know the layout of your pipes...

Circulation pump does not need to be running for bleeding. System pressure alone should suffice for bleeding. "Hissing" when you bled before sounds like trapped air. When you start to get water mixed in, close the bleeder, wait a few minutes and re-open. You should eventually get just water, no hiss, no bubbles.

I'm going to suspect that you have low pressure in the system, so air is not being forced out the top bleeders by water pressure in the system. You should have a pressure gauge (often on the same face as a temperature gauge) on the boiler - what does that read? Normally it should be 20-30 PSI. You may need to add water.

If you can't sort it out yourself, call for professional help sooner rather than later, as letting the system freeze can result in considerable expense.