Check the console log for any error messages, both from Managix and GameMaker. Also note that the progress value is per resource, so even if a texture group has multiple sprites, it counts as 1. Could it also be that the progress is reverting to 0 before it even reaches 1?
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Thanks, Foxy Of Jungle, I'll have a look. I had placed a bit of code to send a debug message whenever progress reaches a value above 0, which has never triggered. I'm going to check the console log, and if nothing turns up, I'll start a blank project to do some testing and get back to you. Thanks for taking the time to respond :