Could this be a result of the bath cooling down in 15mins?
The temperature stated on the plating solution bottle should be held
constant, as James Binnion said.
If you have remaining plating solutions and no way to recover your
cost, then you might as well use them.
I use an electric hotplate with a temerature dial to warm my plating
solutions. Such hotplates need not cost much. I put a 4 quart pot on
it, three quarters full of water, and immerse beakers of plating
solution in that water bath. I regulate the temperature using a dial
thermometer in the water bath. Rio Grande, Otto Frei, Gesswein,
Contenti, and others sell suitable dial thermometers. Allow the
solutions to come up to temperature with the bath.
If you use multiple plating solutions (nickel, gold) at the same
time, and if the required temperatures are substantially different,
use two hotplates and water baths.
The solutions I use require an acid dip between steps. If your
solutions require it, get the acid from your plating chemicals
Rather than sanding your plated work, try burnishing in stainless
steel shot instead. A small tumbler and 5 lbs. of shot will get you
started, for small items. If your barrel tumbler is larger, prop up
one end so all the steel falls to the other end rather than spreads
out. It is nice to go first-class with a large tumbler and $500 worth
of shot, but you can start small and add on as you go.
Keep a record of what you do each time and note the results.