Strategy Tester - "no ticks generated"

 
Would someone please explain, or point me to an explanation of why the Strategy Tester needs to interact with my broker? My broker (MB Trading) frequently brings down their Demo system...why does this prevent me from testing?...I can't seem to test on the weekends either. If I've downloaded historical data, what would cause these problems...or messages like "no ticks generated", or "Requote"? TIA.
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