Good onboarding is a process that must be eased into so the new hire is not overwhelmed and has a clear understanding of the priorities. Yet a 4-6 month waiting period to bring someone up to speed seems way too long! I would strongly suggest that the training process is your problem child.
First, let HR run day one with paperwork, and culture introduction. Move the new hire into their work environment, getting them to their desk, cube or counter, setting up systems, email, phone and the all important tour for the bathroom and lunch room. Introduce them to people, give them a list of people resources with names, phone numbers and emails. Once new hires feel a part of the organization, they are willing to begin learning.
Training should be a combination of skills, policies, operations, tools, and on the job partners. Testing each phase, and practice with feedback. Learning should be in the format (classroom, online, etc.) based on how each area is best learned. Acronyms, as an example, are best learned with a job aid that can be easily referenced.
Managers are critical, but often they are left to their own to figure out how to support the learning process. Give them specific roles, and monitor their participation.
You have an easy problem to fix, and I agree that you need to improve the onboarding process quickly. Let me know if you would like help.