The medicine you find in your LFS or Internet always dosent work on most occasions. I find most of my fish dead when I use them.
Actually, some diseases are easy to diagnose but some are difficult, and sometimes the fish may have multiple diseases.
If they have multiple diseases, you may need to treat those that are more serious/urgent unless you can mix the medications.
Some medications cannot be mixed, else you will kill them.
To really succeed in curing them, you will need a correct diagnosis + the correct medication, correct dosage and correct duration of treatments.
If you overdose, you may kill them.
If you don't treat long enough, the diseases(parasites, bacteria, virus, fungal infections) may return and become even stronger or become immune to the existing medications.
Some medications can also be too strong/harsh and if the fish are weaken from the disease, they may not survive the harsh medication.
Some medications become less effective when the parasites can hide under the fish's mucous(example: gill flukes) and become protected by the fish's mucous in their gills.
Or if you have sand/soil in your tank, the parasites can hide under them and become protected from the medications.
Lastly, if you have plants in your tank, the plants may absorb the medications making them less effective to the disease.
So, for effective treatment, using a bare tank is the best.