Sorry, I cannot answer why there is not a "bill retaiange function" in AP (there may be, but i've never seen it/tried to find a way), but I can tell you what I use. 

I don't know how you are entering subcontractor invoices, so I am just describing how I do.  When entering invoices, the initial invoice amount is the full invoice amount (including retainge).  Then when entering the distribution detail on the bottom half of the enter invoice screen, there is a box for the retainage amount.  To my knowledge (again I could be wrong here) there is not a way to "release" the retainage for payment and begin aging it.  Becuase the retainage is already accounted for when entering the invoice, you do not want to enter in a subcontractor retainage bill as a new invoice, as it will then double up the retainage amount.  When it comes time to pay the retainage, you simply select the amount left to pay in the "Select Invoices to Pay" screen, and it will warn you that the balance remaining to pay is retainge. 

To view open retainage balances, I either pull up a Retainage Report that sorts by vendor and then job, but this will show only invoices that are coded to a job.   I typically use the Cash Requirement Report by Vendor (Reports > Cash Management > Cash Requirement) for the weekly review by the owner.  The report lists invoices by vendor and there is a column for open retainage, if that vendor/customer has any and it was entered in the manner listed above, but it also shows any other invoices that are not coded to a job, but rather just to a GL account (office supplies, equipment repairs, etc.).    You can use either depending on how much information you want on the AP report to be reviewed.  You can alos condition or select ranges to get only certain jobs or certain vendors to show on the report. 

Stacy, thank you for your reply.  I wish they had a release function in AP.  I looked at the report you use, I had not looked at it before, our issue is that some of our subs will start billing their retainage early (sometimes they can reduce their retainage after a certain % complete), and while it shows the retainage on the right hand side, when my boss looks at it, he sees all the subs retainage so it is hard for him to know who has actually billed for it so he can authorize it to be paid once we are funded.  I think I am going to submit this idea as it would really simplify this process.  And since they have one for AR it should be possible for AP also.  I appreciate your feedback!

Stacy, thank you for your reply.  I wish they had a release function in AP.  I looked at the report you use, I had not looked at it before, our issue is that some of our subs will start billing their retainage early (sometimes they can reduce their retainage after a certain % complete), and while it shows the retainage on the right hand side, when my boss looks at it, he sees all the subs retainage so it is hard for him to know who has actually billed for it so he can authorize it to be paid once we are funded.  I think I am going to submit this idea as it would really simplify this process.  And since they have one for AR it should be possible for AP also.  I appreciate your feedback!