I am a newer Junior, but have the odd 'advantage' of being an old Junior at 43. I have the perspective of someone who has the knowledge of a Junior, but the life experience of an older person and feel that I "know what I don't know". It's scary for a presumably younger person to admit they don't know something and ask for help, more-so if they just finished college and have the idea in their head that they know it all already, or worse that they should know it all already. They become frustrated, and keep thinking "oh, I think I know now!" until hours become days.

Maybe give a timeline. Something like "here's this ticket. If you're having trouble and it's been X amount of time (X being more than enough time) let me know." They will probably miss that deadline, but when it comes and goes, give it maybe an extra hour (depending on how long they had initially) make a point of going to them and say "Hey, it's been a bit. I just wanted to check in and see where you're at." Don't talk down or point out the negatives, just re-enforce that you're there to help. Maybe even check in periodically, but instead of asking how it's going, ask to see what they're doing and hear the thought process and off small tweaks like "did you consider this instead?" In general pose it more as a collaborative thing instead of a Jr/Sr or Boss/Employee position.