Some students will need ongoing support with articulation. They should continue to get reminders on mouth shape and how a sound feels while they’re practicing the letter sounds. If students continue to struggle with articulation after ample explicit instruction and practice, you'll likely want to contact the speech and language therapist in your school.
It’s important to remember that students come to school with varying dialects and languages, some of which may not even include the sound being taught. For example, in Spanish there are only five vowel sounds, whereas in English there are about 20. And in African American English, /th/ can sound like /d/, /f/, and /v/, depending on the landscape of the word. Developing a shared vocabulary around articulation and explaining how a sound is made to the whole class for each new sound-letter correspondence sends the message that all oral language systems have value, and that there will be time to acquire the oral language system of school through direct instruction.