Nearly everyone can get an undetectable viral load on ART.
If you do not get a full response it can be for one (or more) of the reasons below.
Adherence. This means checking you are taking the right dose at the right time each day. It might mean you are not following food recommendations.
Choice of drugs. Is the combination strong enough. Some combinations are not recommended with a viral load over 100,000 copies/mL. Also, the higher you start, the longer it may take to come down.
Resistance. Your HIV may have been resistant to one or more of the drugs you used. UK guidelines recommend a resistance test before starting treatment to check for this.
Drug interactions. Some medicines, including supplements and vitamins, can affect how you ARVs work. Speak to your doctor and pharmacist about all medicines and supplements you take.
Side effects. You have to be able to tolerate your meds and they have to be easy for you to take.
Knowing a little about each issue is good before starting ART.
With all these factors covered, if you have a good doctor and you take your drugs carefully, everyone starting ART should be able to get an undetectable viral load.