Sounds like at least one driver is not ACPI-compliant. That may be an oversimplification. There is software out there with components that act as though they were drivers, and those can cause power management functions to be screwed up, too. Examples of this would be some of the anti-virus and firewall software, keystroke monitors, etc.
Got any error messages in the Event Viewer's System log? (Might even be something in the Applications log. I'm not sure.) Too bad it isn't failing to go IN to standby, then at least you'd be more likely to have an error message to analyze.
Long story short, if you don't see or find any obvious error messages it's probably going to be necessary to remove all non-essential devices (and any software that might be acting like device drivers) from the system, then test the system if it will go into standby and resume correctly. If it does, then you add items back one-at-a-time until you get a failure of the function. Then you've got your bad guy. Of course, if the system won't go into standby and resume even when it is configured at its absolute base configuration, then you're talking about a problem with the MB / BIOS / basic devices. That should NOT be an issue with a recent model Inspiron, unless Dell really screwed up in the design. I hate to say it, but, if it won't standby and resume at base configuration, you have to consider the possibility that there's either a basic hardware malfunction or the basic OS install has been clobbered by something, necessitating a clean install. At least if you do that, and the power management still fails, you can be pretty danged sure Dell needs to do some fixin' or swappin'.
- prosaic