Lower back tends to be a stubborn area. Age can contribute - I know a guy who has been competing continuously since the 80s and easily maintains 5% bodyfat off season and cuts down to 3.2% for shows - but has started having issues getting the areas around his kids tight. It's one of those areas that can just be where your body likes to hold fat.
For competition - depends on how much time you have before your show & what you can do. Usually the best thing to do is not panic and continue being meticulous w/ your diet & training and pay attention to keeping your stress and quality of recovery / sleep in check to minimize cortisol.
For fat cutting - what can you do to optimize what you are doing? Are you stalling in your diet and need a refeed? Are you carbing too much / not enough? Good quality fats? Enough total cals? As mentioned above - sprints - can you change up your cardio or your training w/ more HIIT-type sessions?
If you have not cut enough for your show, then it is hard if you're just simply not lean enough. If you're lean enough that you can rely on water manipulation here are some of the traditional tricks:
- topical fat burners
- cardio done in a sauna suit. I've done this starting at like 3 weeks out but be sure to stop by the last week to allow enough time for the water to settle so you don't look squishy. (same idea as why you stop cardio and training by Wed before a show, and keep your legs raised on show day.)
- add some natural diuretics to your diet - asparagus & fish is a classic water dumper
And randomly, lots of people swear by the SQUEEM.