This usually occurs when part of the tool holder touches the rim of the plate before the needle does. Have a look underneath the plate as you raise the needle and you will see where it touches.
- Immediate remedy : Remove the plate, slacken the screw holding the needle (while holding needle with tweezers) and lift it so that the bottom of the needle is just clamped when the screw is re tightened. This raises the needle 2 - 3 mm. Not a lot but often enough to work with.
- Permanent remedy: Pour slightly thicker plates You will need to recentre the needle afterwards but you should be able to dissect with the needle lifted slightly.