Check your title tag is working correctly in HTML would be the first thing.
Bit hard to give a specific answer with that much info. There can be many reasons, do you have an example URL to check?
check the title directly on the website by using the developers tools.
if you see that there is your desired title but in SERP you see another one the reason could be also that Google doesn’t have to show your desired title but might change it and use some content from your website instead because Google thinks that it better matches the user’s search query.
What you can also do is to ask for recrawling your website in the Google Search Console. This could also help rendering your title correctly.