Regarding the orders not appearing in VL10* transaction:
Is the issue isolated to a specific business process - if yes, please share the sequence of business steps.
A couple of things to check - are there any delivery blocks for the order? If yes, check the settings for the delivery block -if it has DDueList = 'X', but does not block confirmations, it will not appear in VL10* but you can still create a delivery via VL01N.
Does the order appear in VEPVG?
Do you get any mesages for incompletion or a problem with delivery groups whan you create the delivery via VL01N?
I believe you have already checked whether there is any filter applied for VL10* and have eliminated authorization problems. 
Regarding orders blocked for credit appearing in delivery due list:
What kind of credit check are you using - simple credit check or automatic credit control?
Please check OVB9 - what is the routine assigned - in case it is not a standard one, please share the logic.
It could be also that the two problems you mentioned share a common root cause.
If both issues can be reproduced in a sandbox system - does running report RVV05IVB for such documents resolve the problem? If yes, I would definitely suggest to check for recent developments resulting in modification of the delivery due index.