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That diode should drop less than 0.5V. If you're getting near 0 after that diode, then it's probably faulty. I'd replace it. You can easily check whether this is the case, even if you don't have a replacement. That diode is nothing but polarity protection, the circuit does not require it. Remove the 9V wire and move it down 2 rows to the other side of the diode. If the circuit works, the diode is bad. Replace it. (If you don't have one on hand, you can use the circuit without the diode as long as you avoid reversing the polarity of the power supply.)
If this doesn't fix it, we'll keep looking.
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