When LDR is saturated with a lot of dark, it has a lot of resistance. This causes Reset to go high. When a light turns on, it sucks the dark out of LDR so it's resistance can go low.
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Problem is, Reset on a 555 works at .7V, so the light has to suck a LOT of dark out of the LDR to get it to work. Even a little residual dark will contaminate the LDR and give it enough resistance to stop Reset from going below .7V. Check the voltage on the Reset line. If it doesn't drop below .7V when you suck the dark out of it, try a higher resistance for R1, like 470k or 1M.