2 consecutive batches with little/no carbonation?

Both batches have been primed with 1.25 cups of DME, bottled, and stored at room temp (68F). One has been in bottles about 3.5 weeks, and the other about 2.5 weeks. Should I uncap them and add carbonation caps, corn sugar, or just let them sit longer? I'm kind of at a loss as to why two batches in a row did this, my first three batches came out great. Any ideas?

No, just move them to a warmer room. 68 is too low. You need around 72F.

They will "bottle condition" at 68, but it will take a longer time.

The use of DME can also take a month to carbonate.

On my last batch, I was in a hurry to have homebrew for thanksgiving, so I siphoned off 1/2 from the carboy. I took it from the top, for clarity. Added 1/3 cup sugar and bottled. A couple days later, I siphoned the rest of it, this time I got a bit of yeast off the bottom, since I hate to waste god beer. I added 1/3 cup sugar, and bottled. 2nd batch carbonated better, faster. 1st batch is OK, carbonated enough for taste and feel, but no head yet- 3 weeks later. I gues I won't be in such a hurry. But then christmas is 2 weeks away, and I just brewed yesterday...