Gutters are fastened to fascias. Fascias are usually the wooden boards that are fastened to the rafter ends.

If the guttering nails are loosening, it is likely the cause is the fascia boards are rotting.

The best fix is to remove the guttering, replace the fascia and reinstall the guttering---dealing with any incorrect installation areas that might have accelerated the fascia rot.

The spikes often start to work out long before the fascia has rotted.
They are actually intended to be aligned with the rafter tails, and nit just the fascia.

You can try and replace the nails with screw in gutter spikes.

The screw threads are less likely to walk out under the repeated shrinking and swelling of the fascia wood.

The big box stores have them sometimes, and McFeelys has nice square drive ones that are not as conspicuous as hex head fasteners.
The square drive recess is barely visible from the ground on one story houses, and cannot be seen on two story houses.

Find a commercial Rain Gutter Supplier in your area.
Here is a link to whats available;http://www.guttersupply.com/p-kstyle-hangers.gstml