How much DNA are you adding to your ligation mix? Too much DNA can be a problem. Try in the vicinity of 20 ng of vector and 1-3 x as many molar molecules of insert. More is not better.
How are you transforming? Again, too much ligation mix can inhibit transformation. 1-2 ul per 50 ul of competent cells. More is not better.
I would not plan on seeing ligation products on a gel. Directly transform, and don't worry about what the gel says.
If you are using antarctic phosphatase, use as little as possible, and limit the time -- pay attention to the units (one of the few times you need to).
You could test the ligation of your insert. After PCR, cut with KpnI (only) and purify. Ligate (only the purified insert), and look for a double length product. Do this again for SacI. If you fail to see a double length product, then there is a problem in the digestion of your PCR product. You already know your vector is being cut from looking at the insert released on double digestion.