Planned orders aren't generating when using PD and HB lot size

Hi all

Here's my scenario. I have an item that is:

MRP type PD

Lot size HB

Min lot size is blank

Max lot size is blank    

Max stock level is 20

Safety stock is 3
There is a previous, and still open, planned order for 3 (I recently changed to the HB lot size/max stock, prior to our last run)

I expected to see an additional planned order suggestion for qty: 17, but the system did not generate any.

I've been reading about PD and HB on here in several threads, and I can't figure out why an additional order was not generated to top up to 20.

Any thoughts would be greatly appreciated!