Not enough yeast

[Smo1]

I'm new to brewing and started a batch of ipa, 5gallons. I only put in 5 grams of yeast when I should have put in closer to 10. The beer starting fermenting but stopped after 3 days and the foam went down. Can I add more yeast on day 3?

 

[CezarRochaNeto]

Hi Smo1, you can add more yeast. Are you using S04?
Check your temperatures as well.