dnd 5e – Players are frustrated when they fail to solve a tough diplomatic problem of how to get them to think outside the box

Problem:

Players got into a diplomatic problem that they know is likely to be above their salary level in terms of difficulty. They spent one session figuring out this problem by speaking to people and performing various charisma checks to convince people they were unlikely to have a business with (roles were average, arguments were not particularly convincing). The party had no big plans, no extraordinary strategies, no clever ideas on site, but tried a very simple head-first dialogue.

This has happened in the past in relation to the fight, and the party had to think outside the box on a recent fatal encounter (one player even said, "Guys, we have to plan more and think less about it, sometimes just hacking and to cut up. "") Now it's a more diplomatic problem that doesn't seem to be as easy as doing a single charisma check and hoping it works.

In the end, the party failed to solve the diplomatic problem (although there will be room to try again with the upper hand in the future), and one of the players said they had not enjoyed the session. Player enjoyment is my top priority. But I also think dnd is best when there is risk, when you can roll the dice, when the PCs don't always win (not that I am actively looking for it).

How can I get the party to act less linearly on dialog related issues?

An example problem:

P: When you try to make a bad person with a lot of influence in the city smart

ON: There are ways to accuse the person, bribe people, spot them to find their weakness, tarnish their reputation, prove their wrongdoing by looking for evidence, and a number of other ways.

I tried to have a short session 0 discussion again about whether they would like to have problems related to the dialogue and they didn't seem to accept it but felt like they tried everything and didn't know what else to do. I also post-mortemed this issue and tried to provide various options that they could have tried, but I feel that the players still feel they have tried and failed and the session was "a waste" (although they're still exp, I still have loot and more action.

A bit of a clue as to how to address this issue, not just "Watch some podcasts for ideas, or read the X, Y, and Z resources on the subject."