burnt tips and yellow withering leaves.

Based solely on the info you provided, I have a couple likely problems you may be facing. If they started to change around week 3 of flower, it's likely P and K deficiency. If they are in veg and the leaves started changing from the bottom of the plant upwards, its a N deficiency. If they are in flower and you feel like your nutes have been right on, it's probably calmag blocking out your P and K. If you've been using calmag since the beginning, stop using it for a week or two. If you're using a bloom booster, it's probably P and K locking out calmag. Stop using the booster. Bad pH can cause this. Test the pH in and out. For soil, you should be between 6.0 and 6.5. For hydro, 5.8-6.3. In both cases, the higher end is for flower-late flowering stages. You could also have just used up the soil nutes that either came in the soil or you mixed in. If you've been going for about 5 weeks or so and haven't had to add any nutrients to your water, then the soil has probably run out of nutes and it's time to add them to your feed. It's a lot, I know, but there wasn't much info to go on. You should know which of these are most likely though so hopefully this helps.