Best place to start would be using the Stratux diagnostic/setup screen accessed by pointing your browser to 192.165.10.1 ... if memory serves. If you're getting data there, i.e., GPS location is accurate (if you have one installed), traffic signals are coming in, it shows "connected", etc., the AHRS chip (if you have one installed) can be oriented (step 1) and calibrated (step 2), then you know the Stratux is functioning properly. AFTER that, check the connection to FltPlan Go. I used mine w/ FltPlan Go on an Android tablet for over a year, and it worked great. Recently, I purchased a subscription to iFlyGPS since it supports AHRS on android tabs (and FltPlan Go does not) and the Stratux works flawlessly with that... traffic, weather, GPS, AHRS all there and glitch-free.
Soo...
1. Power up your Stratux, make sure your tablet is connected to the "Stratux" wifi network, point your browser to 192.168.10.1, and check for signals and configuration settings. Once that's verified...
2. Open FltPlan Go, choose the "External" menu option, and make sure "Stratux" is checked/selected, and that it shows it's "connected."
That SHOULD do it. If not.. get back to me with specifics.. what components you (or whoever built it for you) used in the build (especially which Pi3 motherboard.. the Pi3+ doesn't work with Stratux usually), which version of the software you're using, what tablet (iOS? Android?), and screen shots if you can of anything helpful. I'm sure we can get it working for you.
edit: Another potential place for problems is in the power supply. Stratuxes (Stratuxae? Stratuxi?) are power hungry as Pi3 builds go... two radios, gps, ahrs, etc... so make SURE you're feeding it w/ at least a 2.5 amp capable power supply.