Q: Why cannot I zoom in a map continuously after I zoom in the map to a certain scale?

A: That is because fixed scales have been set for a layer in the map. Generally, map caches only have several fixed scales. 

Solution 1. Map Properties—Bounds—Fixed Scales—Set Scales (you can set visible scales for a layer with fixed scales) .

Solution 2. Map Properties—Bounds—Uncheck Fixed Scales.