Speaking from the old days of experience, using an external effects processor with a Tascam multi-track tape machine. Consider breaking things down to the very the ‘basics’ …typically, you’d use a physical ‘Effect Insert’ I/O routing to send out from an audio track(s) into the effects unit input, and then out of the effects unit output, back into the I/O of your ‘Effect Insert’ connection.
I’m guessing you’re simply routing it wrong somewhere…within your Motu, or within your host software. 1st off, I’d double check that you’re sending & receiving signals properly from your audio tracks as an ‘Effect Insert’, and also from within the Motu’s software control panel, be sure you’re proper routing your s/pdif signals In & Out.
On my Aardvark Q10, I had a hell of a time getting the s/pdif to send & receiving properly. Believe it or not, it wasn’t as intuitive as the audio routing I was used to. The problem I had of not working was all me, within my sound card routing of the s/pdif. Now that I know how, it’s easy. I have no idea of how your Motu control panel is set up, but in my Aardvark Q10, my s/pdif go’s In and Out of digital channels 9 & 10 (coaxial connections), and is shared/duplicated on analog audio channels 7 & 8…that is, the signals in digital s/pdif channels 9 & 10, can be heard/listened to through audio channels 7 & 8 …which is one way to test/verify my signals. My Q10 also has a ‘test tone’ feature to verify signal routing.
With the above, you should verify your signals somehow. You say you see activity in your effects unit, can you verify that there is any signal coming back out of it? My guess is that you’d have to be able to monitor (listen) to the FX’s s/pdif output from your Motu, because it’s a digital signal. Maybe you can try to use the FX’s units analog Outputs, by way of speaker monitors, or headphones directly connected to it to listen for a signal coming out. Does your FX’s unit also have meters to see if it’s sending any signal out?
When you confirm that the FX’s unit is actually sending a signal out, you need to verify your Motu is actually receiving it…which by the way, you should verify that your s/pdif cables are working properly at this time. Are you actually using digital coaxial cables for s/pdif, or regular RCA cables/connectors? How long are the runs? Do you have spares to test against? Can you swap cables for the I/O to compare? I just ordered 2 more new 3 ft long digital s/pdif HOSA cables from ebay, and was delivered within 3 or 4 days.
Back within your host software, verify you’re sending & receiving from your Effect Insert channels properly. I haven’t really used much external effects since I’ve been doing computer recording mind you, and as for buses, I’ve stuck to the very basics without exploring complicated signal routing (only what I need), I’ve never used many features in Cubase in this area, so I’m not the most experienced when it comes to Cubase routing & buses. The purpose of my reply is only to make you think about what you’re doing and how you’re doing it, and to consider to ‘basics’ for troubleshooting. This is how I approach things when troubleshooting, taking it ‘point by point’ and verifying things along the way. Somehow, you’ve lost it along a point…or two 
Just yesterday, I spent a handful of hours experimenting with routing digital s/pdif signals in various ways between 3 computers via their audio interfaces…it really get’s confusing. By the way, I use computers as my ‘outboard gear’ these days.