This does not look like a great design missing so many details such as the light beam interruption and how stray light is blocked is very important.
You have an obligation to show what values you used and what layout and voltage vs light measurements. Then actually explain your problem with a question!
So I decided to simulate here on Falstad
On this, if you simulate a slight loss of light by sliding the pot to the right the LED goes ON ( Alarm ) and latches with Trig Pulled up on pin 2 . Almost any value works for Trig pullup, 1k to 1M
Then you can click on my SPDT switch to reset the alarm. Then slide pot back to left to simulate light detected and repeat.
The R2 value determines your threshold of detection, with the Control Reference exactly 2/3 of Vcc.
Edit 5:06 pm
Now seeing your values, looks like R2 is far too low and requires a lot of light to trigger. Increase it until you trigger then back off a bit just to simulate loss of light beam.