Or a fixed bar, or the blade of the tool is too thick to get in the gap, or you have shoulderless spring bars in there, or the fabric doesn't have enough give you allow you to slide the spring bar back enough to clear.
the blade of the tool is too thick to get in the gap
That sounds about right.
@Sanddancer started
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Or a fixed bar, or the blade of the tool is too thick to get in the gap, or you have shoulderless spring bars in there, or the fabric doesn't have enough give you allow you to slide the spring bar back enough to clear.