In Game of Thrones Season 7 episode 6, some extra Rangers in a small party are killed. Thoros has revived the dead in the past using magic. Given that the mission was really important, and they only had a small party, why didn't he attempt to revive the extra Rangers after the encounter with the Polar Bear? In earlier episodes, we have seen Thoros revive someone else, and it only took a few seconds.
Thoros can't just bring anyone back. He makes this clear in a conversation with Arya.
“Could you bring back a man without a head?” Arya asked. “Just the once, not six times. Could you?”
“I have no magic, child. Only prayers. That first time, his lordship had a hole right through him and blood in his mouth, I knew there was no hope. So when his poor torn chest stopped moving, I gave him the good god’s own kiss to send him on his way. I filled my mouth with fire and breathed the flames inside him, down his throat to lungs and heart and soul. The last kiss it is called, and many a time I saw the old priests bestow it on the Lord’s servants as they died. I had given it a time or two myself, as all priests must. But never before had I felt a dead man shudder as the fire filled him, nor seen his eyes come open. It was not me who raised him, my lady. It was the Lord. R’hllor is not done with him yet. Life is warmth, and warmth is fire, and fire is God’s and God’s alone.”
From this, it is clear that it isn't Thoros that's bringing people back but R'hollor.
@Edlothiad's answer answers why it probably wouldn't have worked if he tried but there's also another very obvious reason.
He's just been mauled by an undead bear
I have a detailed answer here as to who dies and when they die but I'll give the short answer here. The attack of the wight bear is the first time the merry men loses someone and is also the first fight they have. At the end of the fight Thoros is mauled by the bear. After that he needs help walking and his incredibly fragile, indeed dying a few hours later. It's likely he's having trouble concentrating too considering blood loss, even with Beric having cauterised the wound. As such I doubt he even thought about doing it.