Systematic Bug Investigation

Help me systematically debug this issue:

SYMPTOM: [What's happening that shouldn't, or not happening that should]

EXPECTED BEHAVIOR: [What should happen instead]

REPRODUCTION STEPS:
1. [Step 1]
2. [Step 2]
3. [Step 3]

WHEN IT WORKS: [Any conditions where it works correctly]
WHEN IT FAILS: [Any conditions where it consistently fails]

Please help me:
1. Identify the most likely causes
2. Create a debugging plan to isolate the issue
3. Suggest specific things to check or log