a little disaster has struck

[sheldon1982]

hope someone can help

i was plastering under the stairs last night , when i was polishing the plaster a small amount of it just fell away when i tried to put some more plaster into the hole it just got bigger so i have left it to dry overnight.
i now have a small hole , it looks as if the plaster isn't even touching the boards underneath??? the boards under the plaster are a little bouncy. what do i fill the hole with that will give a good finish????
what if when i fill the hole the plaster just cracks and the hole gets bigger?????

confused as the rest of the plaster is sound..

thanks

 

[gday2uk]

Just use polyfilla or similar.
If the hole get bigger etc. then you are f**ked and will have to start again; this time properly preparing the background you are plastering.