You should try longer exposures. For stars and globular clusters 5 minutes is usually OK, unless you are looking for very dim stars, as a general rule of thumb you can see magnitude 15 with a 5 minute exposure and up to magnitude 17 with a 15 minute one. Of course it depends on a lot of other factors, like light pollution, Moon, clouds and so on. I can consistently see magnitude 17 stars from a Bortle 7-8 area with 15 minute exposures.
For galaxies and nebulae, except for the very bright ones like M42 and Andromeda, you should consider longer exposures in the 15 minute to 60 minute range.