Why did my iron nails not dissolve in hydrochloric acid?
It seems like there is not enough acid on the nails.


Perhaps a depleted layer is forming next to the nails. To avoid that issue, use a magnetic stirrer or provide a source of vibration.
There might not be enough $\ce{HCl}$ for the large amount of iron in the nails. Check the total amount of $\ce{HCl}$, given the volume and that ~2/3 is "other ingredients" (likely water). Weigh the nails. Calculate the number of moles of  $\ce{HCl}$ needed to react with the number of moles of  $\ce{Fe}$. Is the product  $\ce{FeCl2}$ or  $\ce{FeCl3}$?
Have you removed all the grease (from the manufacturing process) from the nails? A little soap or detergent and rubbing may be needed. 
Could there be some difference in the nails you used? If some are galvanized (zinc coated, to prevent rust), then that would affect the reaction.


Let us know how you do: we learn more from our mistakes than from successes.