Hi Iain,
The padlocks are to indicate (depending on which direction the arrow goes) that a particular measure is locked to a previous measure and should stay with that measure either on the same system (left arrow) or the same page (up arrow). I'm recommending that using the "Specify number of bars" command should allow you to either keep existing locks (such as you had) and operate on the rest of the score, or it should allow you to over ride already made changes and set the whole score to the designated number per system. Task 2169.
For playback, there is a slight chance that if you have an older machine and Composer was in the middle of an autosave, that it may not have begun playback immediately. That would be sort of the equivalent of hitting a gnat with a pea shooter from a dozen yards - not probable, but possible
If the problem arises again, please let me know.
ttfn,
Sherry