As I said in my above post, Kazakhstan, Kyrgyzstan and Tajikistan are part of Russia's CSTO. They will never say No if Russia says Yes.
The reason why Iran was not accepted earlier is due to two reasons:
1) UN Sanctions (which will be lifted now)
2) Lack of agreement on using Caspian Sea resources (which has also been resolved now)
Now Russia has not only supported Iran's membership publicly but it has asked other members to do so.
I don't care what you say, Tajikistan blocked Iran from becoming a full member in the SCO in 2017. And please stop pretending that you know news or foreign affairs related to Iran better than Iranians.
Kommersant: Tajikistan blocking Iran from full SCO membership
Foreign Ministers from member states in the Shanghai Cooperation Organization (SCO) will hold a conference in New York. This will be the first meeting with the participation of Indian and Pakistani top diplomats, since both states became full members of this organization this past June. According to Kommersant, Russia hoped to see Iran at the negotiating table, given that after the removal of UN sanctions, Tehran has the right to apply for membership in the SCO.
However, Tajikistan is accusing Tehran of supporting a banned Tajik party, so it is stonewalling Iran from getting SCO membership. Experts interviewed by the newspaper do not rule out financial issues behind the unexpected spat between the two previously closely cooperating states.
Moscow believes that the international role of the SCO will only strengthen if full membership is granted to Iran, which currently has observer status in the organization, the newspaper wrote. That said, there hasn’t been any formal obstacles to changing Tehran’s status since the removal of the sanctions over a year and a half ago.
However, a recent documentary filmed at the request of the Tajik Ministry of Internal Affairs accused Iran of orchestrating a string of terrorist attacks and contract killings in the late 1990s. Tehran rejects all accusations, noting that the information contained in the documentary is not true.
According to Andrey Kazantsev, an expert at the Valdai Club, director of the Analytical Center of the MGIMO Institute for International Studies, the belief that the current spat with Tajikistan is due to financial reasons is "widespread" in Iran. "But I would refrain from calling it anything more than rumors. There is no official evidence, and the charges are very serious," the expert told Kommersant.
Kazantsev suggested another possible reason for the discord between Dushanbe and Tehran is the growing influence of Saudi Arabia, a long-standing adversary of Iran, on Tajikistan’s policies. In 2016, Tajik leader Emomali Rahmon visited Riyadh, calling Saudi Arabia "an important partner" for his country in the Arab world. This spring, the media reported that the Saudis are going to build a parliamentary complex in Dushanbe, which required tearing down several buildings in the center of Dushanbe, including the Iranian embassy, Kommersant wrote.
Source:
https://tass.com/pressreview/966496