Marketo Assets Not Appearing in Dropdowns?
If you ever have to use assets (forms/LPs/etc.) in your SL triggers/filters that weren't created in Marketo but were created in a separate platform that is integrated with Marketo, then you would not see the asset in the drop-down until Marketo saw an activity for it.
The best example of this is LinkedIn Lead Gen Forms integration. As you'd probably know, once set up, the LinkedIn Lead Gen integration would allow Marketo users to trigger off/filter people based on their lead gen form submission activities.
NB: These forms are created in LinkedIn and not in Marketo, and hence Marketo won't see new lead generation forms that don't have any submission activities on them.
When people submit their information in a LinkedIn Lead Gen Form, that information is pushed to Marketo immediately, making the form available in the "Lead Gen Form Name" drop-down in triggers and filters.
So next time if you're not seeing your newly created LinkedIn lead gen form (or any asset created externally) in the drop-down in Marketo, don't freak out, just make a test submission/visit on the form/LP so that activity gets pushed over to Marketo and you should be good to go.