Despite of using anticoagulant from years, my mom was affected with DVT

My mother had undergone aortic valve replacement surgery in the year 2004. She has been prescribed with anticoagulant (acitrom) post operation.  From then onwards she is been monitoring under PT/INR report and based on that anticoagulant (acitrom) dose is decided by our doctor. Initially it was 2mg/3mg alternate day and later on changed to 4mg/6mg (alternate days) (based on PT/INR report).
Despite of using anticoagulant from years, last month she was affected by Thrombosis in left leg.  We rushed to the doctor and under his guidelines we did PT/INR test and the report came abnormal. We did Doppler Ultrasound test and the report shows Clots in the leg. He prescribed Clexane injection (enoxaparin -low molecular weight heparin) 8 doses. Later we again did Doppler Ultrasound test and the report shows very minute clots (Doctor said no need to worry). But after one week my mom had again pain in the left leg calf.  We consulted vascular surgeon regarding the situation.
Vascular surgeon suggested to wear Stockings for a minimum of 3-4 months and reduced the dose of anticoagulant (acitrom).  Now my mom has no pain and she is able to walk.
Since affected my mom is using medicines for diabetic and cholesterol from past one and half year.
Medicines:
AVR post surgery:  Acitrom
Diabetic: Gluconorm G1
Cholesterol: Xtor-F
Blood Pressure: Losar H
Bronchitis Asthma: Deriphyllin
Multivitamin tablets: Supradyn
DVT: Acitrom

1. Can anyone please let me know, what might be the reason my mom was affected with DVT, despite of using anticoagulant from years.
2. What are the necessary exercises need to be done for DVT and precaution measures for clots developing again.