Marketpress Paypal Express failing on multisite installation

I have a multisite installation with Marketpress running. ALL of these stores are running from the one plugin

Transactions via Paypal Express (the only gateway) are failing on and and possibly

However, payments are not failing on

What the failure looks like:
Checkout takes customer to Paypal, customer enters all details payment appears to complete successfully but then no order is received and no payment is recorded either in Marketpress or Paypal.

I would generally expect this to fail also in as this is exactly the same setup and installation, but sales are working there fine.

This one has me stumped, and I'd really appreciate some urgent help on this please.

All plugins at the current version. Running 3.6 although this may have been happening for some time, prior to 3.6 installation.