What are the causes of permanent eyelash ptosis? And how to repair it?

The insertion of the levator aponeurosis tendon into the tarsus and the upper eyelid platform skin (makes the upper eyelid crease) supports the upper eyelid platform skin and the upper eyelid lashes.  With the loss of the support ( you have a white line levator disinsertion ) the lashes hand down and shade the upper  part of your cornea and make your eyes look dull and lifeless.  You need an anterior elevator resection ptosis repair with an anchor blepharoplasty with  lowering of the upper eyelid crease.  As I have stated before, this will make a big difference for you.

Eyelash ptosis is sometimes related to eyelid ptosis, and often related to a condition called floppy eyelid syndrome, and sometimes both conditions are involved. Laxity and stretching of the eyelid tissues, causes the eyelid tarsal platform to lose tone, thereby weakening eyelash support causing them to hang down. In addition, sometimes excess skin will also weigh down the lashes.  A comprehensive evaluation is necessary by an Oculoplastic specialist.

Thank you for your question!
Eyelash ptosis can be the result of a number of factors including prior surgery, advanced dermatochalasis, orbicularis weakness, and mechanical effects on the eyelid. 
Every eyelid is different; however, there are some tried and true means of correcting lash ptosis in many situations. One approach is to use sutures to rotate the lash line at the time of blepharoplasty. This is a fairly straightforward technique and is one that is commonly performed by oculoplastic surgeons. Consider seeking a consultation with one!
Best of luck! 

Thank you for your question.  Without photos, I cannot comment on your exact situation.  Causes of ptosis include  aging, infection, surgery, mechanical (from mass or lesion), and genetics, among others.  Follow up with a surgeon experienced in eyelid surgery for a comprehensive examination, and to learn more.  Best wishes.