My estimative guess.
1) The impeller casing could have an air built up (air lock). This would accumulate over time and the impeller will continue to be spinning in the air lock. No water movement will occur.
2) Somehow, the impeller gets loose and isn't operating at all. There maybe some small parts that the impeller shaft sits on and that you may have missed installing it back on during maintenance.
Just guessing. HTH.
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