Not able to get vison corrected to 6/6 even after using specs.

Did you consult a qualified and experienced eye doctor? If so did you go for a second / additional opinion in this regard?
I would suggest you consult an eye specialist doctor in a reputed and established eye hospital. If they say they cannot correct the vision and diagnoses the real issue, then additionally you may consult an Ayurvedic eye hospital.( For example Sreedhareeyam eye hospital in Kerala). Or any god Ayurvedic hospital or doctor in your area.

However you can take Triphala tablets in the meantime. Triphala is known to aid eye health and improve vision. But it is suggested you consult a qualified doctor before taking up any self medication. Take green leaf vegetables recipes in your food regularly.

Relating to your current position, you need to meet eye specialist for the correct diagnosis in your case. If required, you will be investigated by the team of doctors for assessing the internal conditions of eyes. To correct your vision, sometimes eye surgery is need to restore normal vision ( 6/6) if such treatments don't respond in correcting your vision.
You have the option of visiting Ayurveda Specialist who would undertake your Prakriti such as Vata Pitta, Cough Prakriti or Cough Vata Prakriti and this will form the basis of investigation.
There are a lot of medicines in Ayurveda which can heal up your present conditions. A few hew Rasayanas could help you in alleviating your disorders such Saptamrit Loh, Nawayas Loh, Triphala Churna where these medications will be administered internally or washing the eyes with this Churna dissolving the concoction in water and washing the eyes thoroughly with this Rasayana depending upon your case. However, it will take time for the restoration of your normal vision taking even a year. You take the consultation of a reputed doctor so as to have perceptible results.

The eyes are a very delicate part of our body. So, It is a great need to take care of it. As mentioned above that your vision is not correct after wearing spects So, you must go to an eye specialist who will detect your disorder and then prescribe medicine. Because this problem can handle doctors only. As homemade remedies are concerned, you should do little eye exercise and take leafy vegetables like carrot, spinach etc. Try to walk on green grass with a naked foot in the morning time. These are little activities which would improve your weak eyesight.

A correct answer to your question can be given by a qualified ophthalmologist with good experience. You should go to a good eye institute like L V Prasad Eye Hospital / Sarojini Devi Eye Hospital or Sankar Netralaya and meet the concerned doctor. Then he will examine you and tell you the suitable way forward. But trying things without proper understanding is not a correct remedy.
There are some surgeries and a suitable one can be suggested by a qualified doctor only. Some may suggest some glasses or contact lenses also. Some medicines may be prescribed by the doctor.
In addition to the above, you can take the following precautions so that there will be some improvement.
1. You should have good food habits. You should not eat junk food. Eat more and more leafy vegetables and fresh fruits. Also, eat nuts and seeds. Consume some eggs daily. Also, eat some citrus fruits.
2. Reduce your screen time. Spending more time on screen may further enhance the problem.
3. Protect your eyes from sunlight and give sufficient rest to your eyes.
4. I hope you have no smoking habit. If you have please stop. Smoking will cause vision problems also,
5. There are some Ayurvedic medicines which may provide you with some relief.
But you should go to a good eye specialist and take his advice and go ahead.

Generally when we get an eye test then depending on the weakness that is there in our eyes either in the form of myopia (short sight) or hypermetropia (long sight) the doctor will prescribe a spectacle to us. If the spectacle is not able to correct our vision to 6/6 then it means that there are some deformities in our eye either in form of a mechanical defect or a biological deficiency.
At this stage it becomes imperative to go to an expert opthalmologist who will go for more tests and after complete testing will come to a diagnosis in this matter. He can suggest some laser corrective measures or some special lenses which might help in the situation. He might also prescribe some supplements to strengthen the eyes.
There are many types of aberations in the eye and it could be a combination of spherical, cylindrical, or alignment of the eye ball in the eye socket. Based on the true picture of these deformities the expert doctor can advise the further course of action.