swimming pool film on water surface

I Took on a house with an inground pool last year. Lately there’s a a kind of “film” on the surface which I’ve been unable to clear after vacuuming the
pool, backwashing/rinsing and evening putting a “scum ball” in the skimmer. Have tried “shocking” the pool with chlorine granules. Two concerns, maybe linked ? 1) Although when I move the basket I can put my fingers down the skimmer hole and there’s suction, it doesn’t appear to be very strong (don’t really know how strong it should be) and the chlorine tablets seem to take ages to completely dissolve (weeks if at all - tend to find several in the skimmer basket at once). Don’t think it’s the cover and the chemical balance seems almost perfect. Any ideas anyone ?
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