Trouble with sound output

I’m using 3.5 on MacOs 10.15.
I’m having quite a bit of trouble with sounds not outputting on projects when they are opened. The sound did work on these projects previously. However, when they are re-opened say the next day there is no sound.
I primarily use Spitfire Audio Labs or BBCSO Discover. But this happens even when using a single instrument, for example using Lab Soft Piano .
It is intermittent.
I can tell there is MIDI coming from my keyboard to Dorico, I see the green light.
If I use the mouse to click on a piano key in the LABS plugin I get sound.
I did turn on my MIDI keyboard before opening Dorico.
If I open Logic Pro and use Labs or BBCSO, everything works fine.
Its such a simple setup, I’m not sure what else to troubleshoot.

I assume that it then has something to do with your endpoint set or the proper sounds not loading into the VSTInstruments.
If you get into this situation again, make a backup copy of your project and then in Play mode, choose from the menu ‘Play > Playback Template’ and then the HSO template. Does that then play back properly?

I’ll try that next time thanks

It happened again. Applying the playback template didn’t work.
The situation was I was on the road with my Macbook and left Dorico open when I got home and reconnected to my studio setup. Later I turned on my midi keyboard. Dorico wasn’t responding so I force quit it and reopened it. Dorico asks if I want to recover the last project I was working on and I click ok. Then there was no sound. I tried reapplying the HSO template but ther is no sound when pressing a key on my midi keyboard. There is sound if I use the mouse to click on a piano key in HSO.
Then I quit Dorico and don’t save the project, reopen it and the sound works with the midi keyboard.

When you force quit Dorico, you also need to kill the background process VSTAudioEngine, that is the audio engine of Dorico. You need to kill it using the Activity Monitor utility or a Terminal window. Because the VSTAudioEngine is in an undefined state if you force quit Dorico and next time you start Dorico it connects to the still running audio engine, but with the effect, that it might or might not work. Chances are less likely to work correct, so it is better to bring both, Dorico and the VSTAudioEngine into a clean state.