I've had the same problem, I think. The bags are dry on top and around the sides for maybe 2 inches or more and still moist around the plant. When you try to water them it just goes to the sides and leaks out.
This is what I did even after being told it wouldn't work. I use 7 gal fabric bags with Super Soil.
If you spray the top of the soil to moisten it and wait for a little while for it to soak in enough to get it damp the water will soak in easier. In order to git even moisture in all the soil I slipped the pots into 13 gal kitchen trash bags. They fit almost like skin. This will slow the evaporation through the sides. Use a moisture gauge to check for consistency. When The moisture starts to even out just pull the bags down. When you water after that pull the bags up. Use your finger and make a ring about a inch deep just less than half way from the side to the plant. Spray it. Water it. Check it. Pull the trash bag down when the moisture is fairly even.
I have also found that if I don't do something like this there will be a 5 gal root in a 7 gal bag. The roots don't want to go where it's dry.
This works for me. Might work for you. Btw, I was cautioned a couple of times about mold between the fabric and plastic but haven't had any. If it gets damp it would only be for a short time, from a couple hours to a day.