The problem is caused by the sessions screen not updating with changes to the classes screen. This can be checked on the report – Reports | Cluster/Session mismatches. If such changes are made on the classes screen, the system does not necessarily know what corresponding changes need to be made on the Sessions screen. e.g. If a teachers number of periods is reduced from 4 to 3, from which session should they be removed?
To make the correct adjustment on the sessions screen, find the block concerned and click on the appropriate cell in Groups and Clusters section. The Attributes column should now show the imbalance. In the sessions column, right click will take the wrongly assigned teacher off a particular session and then the correct teacher can be dragged onto that session. Once the balance is correct, the block can be rebuilt on the blocks screen.
A much better way to handle teacher replacements is not to make the changes on the classes screen but to use the edit staffing function on the blocks screen.