this is a frequently asked question. We have a search function here.
Have you ensured that your machine is accurately rectangular (“squared”) and coplanar (“not twisted”)?
Also Onefinity provides a help file which deals with many kinds of blocking of an axis:
I would check all points one by one to see why your carriage does not move anymore in the middle of the axis.
There are three causes that are not mentioned in the document above:
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Should your machine not be properly rectangular (“squared”), the Y movement can block.
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And the second possible cause not mentioned in the document above is: This mysterious issue some time ago.
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Electromagnetic interference (EMI), as e.g. mentioned here.