What bass is it? Does the truss rod adjust at the headstock end or the body end of the neck? Assuming its an allen key at the headstock end, I'll repeat what itu said above, ie it is of the utmost importance is to ensure you're using the correct tool. A metric allen key that *seems* to fit nicely in an imperial nut (or vice-versa) can damage the nut, rounding the hole making it near-impossible to get it loose without specialist remedial tools. The short allen keys you get with most instruments that look like the ones in poundland aren't up to much and don't give much leverage, so I tend to use long handled versions such as a bondhus t-handle hex drivers (which I believe Fender rebadge and include with some of their instruments). This gives more leverage and makes adjustments much easier. I also label each one with the spec if it isn't stamped into the handle and which instruments I can use it on.