Hello @LynnR978 ,
This can occur when there's a significant amount of upload latency on the user end while trying to move blocks around. My recommendation would be to first try clearing your browser's cache to see if that alleviates the issue. If it doesn't try using the builder on another browser, or through Incognito / Private browsing to try blocking out any unneeded addons / plugins. The last troubleshooting step would be to turn off your internet router for 10-15 seconds, then turn it back on.
If none of those work, please let us know and we'll try some more advanced troubleshooting.