Drooping - could it be underwatering?

I have some small plants (about 12") that I just switched to 12/12. The leaves on one of them more than the other seem to be drooping a lot. I know for sure it isn't overwatering because I have been watering every 4 or 5 days because it seems like the soil stays wet for a long time, and I thought they were suffering from overwatering because of the droopy leaves, but it seems most people water every 2-3 days. The plants are in plastic gallon containers with the top cut off (I just put construction paper over them after learning the roots don't like light.

I watered on friday and the soil seems dry so I think I'm going to water again today. I just got a ph meter so i'm gonna ph the soil again. My question really is what are some signs of underwatering, and is it possible the soil stays moist for as long as I have been thinking. (Generic cheapo potting soil and perlite mixture)

Do they perk up after watering them?

Its hard to offer any real opinion without a picture.

What kind of drainage do you have in the bottom of the milk jugs,without proper drainage the soil can be mush on the bottom & dry as a desert on top,not a good scenario,make sure you have ample drainage,you'll need atleast 4 holes about the size of quarter.

Also,dont waste any money on a cheap ph meter,if you cant afford a real good meter dont bother, buy a ph test kit for your water at any fish store or wallmart,let your water sit out for 24 to 48 hours before testing then adjust the ph to 6.5 & give your soil a good flush right after you adjust the ph,you can add a splash of peroxide to the water to help get some oxygen to the roots.

Seeing that your just using milk jugs take a razor blade & cut a decent size hole in one container about 3/4 of the way down,this will allow you to see exactly whats going on with the moisture in the soil,you can put the plug back with duct tape.

I have a couple holes on the bottom of each ranging from as thick as a pencil to a dime. Guess I'll be making those a bit bigger, and great idea about the little viewing hole! I'm trying to get pictures up here but am having slight difficulty

Here they are

pH is 5.5. Very well could be the reason. I'll update after the pH issue is settled.