It is, as others say, difficult to advise but in your position I'd be pretty annoyed. Assuming that they are not officially a co-tenant, and that I'd given them what I felt were sufficient warnings that lack of action from them will lead to withdrawal of sharing rights, then I'd tell them it wasn't working out for me.
Either way, I'd decide what to do before they invest too much time or resources this season; or, give them one last chance with a clear warning about the expected level of input, and see what happens this season.
If the plot is too big for you on your own, is there a possibility of formally reducing it by half, for example?
PS Welcome to the website