Too many subplots. One review of my debut novel, Can’t Live Without, complained that it had too many subplots, and I think she had a point. (And I’d already cut two!) Sometimes writers can add too much to a novel, afraid it will be boring if the focus is too narrow. If your central premise is strong, there is no need to worry. Subplots can be great if used well, but too many and the reader gets tired.
I've been working with some Fiction Frenzy writers. Their plotlines sizzle, then fizzle -- too much happening to people we don't know, or what to know.
The above tip is great advice. Keep your focus.
If you MUST keep a sub-plot, turn it into book #2.