Is there a factor that modify your chance of stealing something?

When you have the thief in your party you can steal items from some pnj. You have a given pourcentage of chance to successfully steal each available item. I've noticed that for the healing grape at the moment I have a 80% success rate for the regular one and 8% for the upgraded one (in french it has a (M) next to it, for Medium I guess). I didn't see a difference when standing behind or in front of a pnj. So is there any factor that change your chance of stealing an item ?

Your success rate increases as your thief's (Therion's) level increases.

There are also, in some towns, people that, if you scrutinize (with Cyrus) or inquire (with Alfyn) will give you an increased success rate. More specifically, when you do either of those actions you'll see "Thieving Tips & Tricks" in the "Information Gleaned" section of the dialog box.

https://gamefaqs.gamespot.com/boards/204212-octopath-traveler/76889313 lists all the towns that have an NPC that'll boost your steal success rate:

• Bolderfall - Townsperson (Near Entrance)
• Noblecourt - Townsperson (Near Book-loving Girl)
• Grandport - Elderly Woman (Inside the House with Blocked Entrance)
• Marsalim - Townsperson (Near Entrance)
• Northreach - Townsperson (Inside House, Higher Star rating)
• Riverford - Right Guard (Inside Tavern)

You can also kinda cheat by constantly restarting the game. In the Octopath community doing that is called "save scumming".