I have some experience with this and apologies for being late to the gun.
Several things that I found that you need to do to make shihonage effective.
Against a static opponent when his balance is not broken it's almost impossible to do shihonage without atemi. However just by stepping back and extending the hand forward and bringing him out of his balance can help you create the entry. You need to break his balance first BEFORE attempting the entry. You can do this slowly provided he's not giving you full resistance.
There's a torque on the wrist that you can also do while entering and lifting his body up that gives you the ability to enter safely.
Now from your post I can gather that your friends have let you entered but it's more of you being unable to complete the turn without them resisting. This is probably due to something you're missing in the technique. If you're not doing shihonage dynamically, you can still apply a mini arm-bar type ish to lock him in before you turn. This locks him in position and gives you time to properly adjust your body to take away the space between the both of you. (I wouldn't recommend this in actual technique but I usually apply it a little bit even when doing it quick just to give that added effect). This also prevents him from spinning out of shihonage if you're keeping the technique tight.
The turn has to be relatively quick but make sure you're turning at the hips and keeping your hands always in front of your head. Drop your body without compromising your stance. If you do that, once you turn, uke will be off balance without you having to even cut down.
You can also play around with Stanley Pranin's suggestion on a different shihonage grip that gives you more control (especially in non dynamic attacks).
Here is also a quick video of me doing shihonage with the revised grip. If you can slow it down you can see how I turn. It's not perfect since I was more focusing on the grip but you can get the idea. It's not as tight as I would normally do it due to the revised grip I'm using which already automatically locks his arm out. However these things of course need to be felt :P