Reasons and excuses are a bad idea. Honesty, expressed in the appropriate manner, is far more effective. Just tell them, quite honestly, precisely how you feel when they hold on to your stuff (eg disappointed in them, irate, whatever is exactly true) and why (because it looks like you don't respect my stuff, or because I don't have it when I need it, whatever is exactly true). Unless they are sociopaths or have other personality disorders, this is a very effective way of getting people on side, which is why it is taught in management schools.

Then in future set a time limit for bringing them back, and enforce it. That way, you will definitely have your tool when you need it.