Why 1998 Subaru Forester turn signal lights not workin although you have replaced all bulbs?

A likely cause is that the flasher has failed. On my 1999 Subaru Forester this is an inexpensive (~$10.00 from my local discount auto parts store) and small part.

What I found was that obtaining the replacement part was easy and actually replacing it was challenging. The challenge is that the flasher is located near/on the firewall fairly high above the driver's footwell. For me this meant being in the driver's seat upside down with my head near the pedals, my feet on the back of the seat, and my arm snaking up through the wiring to access the flasher.

Best of luck.