What should I do if I am completely stuck on a bug or backlog item?

I’m a junior dev and I feel like I’m out of my depth. Some backlogs or bugs I can complete quite well but they tend to be simple changes. However, with some larger changes I struggle. Sometimes I find it difficult to even understand the bug and replicate it within the dev environment. I feel like I’m under a lot of pressure. I don’t want to constantly have to go to the senior devs for help as I feel like I’m a burden. Currently working in my first job out of university and working within this company for about 3 and a half months.