Over time, the wood of the shamisen can suffer from moisture and may break, sometimes it just wear and tear down but others can be more serious.
Perhaps you just may need an adjustment for your itomaki.
Any how i can give you a solution cause every malfunction can be fix it!

Here you can see some examples:

Neck repairing and polishing
Slippery Itomaki
Neck Break
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Fill in this form explaining what is wrong with your shamisen and I will send you a quotation as soon as possible.