Separate fabric performance from fitting
A fabric can block light through its weave while bright lines remain around the window. This is why replacing only the curtain fabric can leave the original complaint unchanged.
Use daylight to identify the path. A glow across the panel suggests fabric or lining. A sharp line at an edge suggests coverage or installation.
The four places to inspect
Check the track above the window, each side, the meeting point of the panels and the finished length. Enough fabric width also matters, because a curtain stretched almost flat will behave differently from one with fullness.
If the room needs darkness for sleep, explain that goal to the fitter. A decorative dim-out curtain and a carefully fitted blackout system are not the same brief.
Before spending again
Bring a straight-on photograph, the track width and the drop from track to floor. Ask whether a fitting change can solve the problem before remaking the curtains.