Modern systems need more maintenance support ammo ect., and in combat need that maintenance, ammo, support, even more so effectiveness falls dramatically.
Now I am going to go a bit further with this, from some personal knowledge.
At the current time, the US military is at one of it's lowest levels of readiness that it has been in for decades. And this goes to the Maintenance-Preventative Maintenance circle I mentioned earlier.
Now every week all active duty units in the military perform Preventative Maintenance (PM) on all equipment that requires it. Be it a tank or cannon to a HMMWV or PATRIOT Launcher. We fill out in the Army the DA-5988 or DA-2404 (ERO or Equipment Repair Order in the Marines) with everything wrong with our equipment, and then the S-4 makes the request for the replacement parts.
This can be everything from a windshield wiper for a truck, to a new barrel for a gun. We notice it is broken-defective-out of tolerance, and we request it to be replaced.
Now say 7 years ago, we generally got it as fast as it could be acquired and shipped to us. Allowing for certain priorities (units overseas got them first, as did units about to go overseas).
In the last 6 years, we have seen things that would have been replaced in days take weeks, and weeks take months. And sometimes it has taken even longer. This is because less money is being spent on replacement parts. And for units that are expected to remain stateside for 2-3 years 9say they just got off of a deployment), they may wait 5-6 months for the replacement parts.
In 2008,we had the previously mentioned Elevator Actuator on a PATRIOT Launcher in my Battery fail, and we had 2 replacement units in 3 weeks (there are 2 per launcher, both are replaced at the same time). In 2011 we had an Elevator Actuator fail on my PATRIOT Launcher. When I left in 2012 (7 months later) they still had not been replaced.
And failure to fix equipment can cause more failures. We had one launcher in my battery that had a faulty Generator. It was throwing excessive voltage, but we could not get the authorization to replace it or dismount it and send it in for depot level maintenance (no money in the budget). So we continued to use it, we had no choice. Then one day it had a critical failure during a PM cycle, the generator caught fire (extinguished quickly), but the voltage spikes caused 10 circuit boards in the launcher to fail. I would guess that was probably $500,000 in damage, because of a $35,000 generator that was not replaced when it should have been.
Probably the biggest challenge our military has is that the PM and replacement for much of our equipment has been put off for 6+ years, because of budget concerns. Each and every unit in the military works off of a budget. Once you reach it, no more maintenance (replacement parts) is given to them. And many times the replacement parts are simply not in the logistical chain, because the military as a whole has not been given the money to purchase them.
Some regular PM items (fuel filters, air filters, tires, etc) is readily available. These are expendables, and are expected to be replaced regularly. But more unusual items (outrigger pads for PATRIOT, gun barrels for an M1, etc) are in short supply, because these have to be ordered by set amounts and currently they are trying to only order what is absolutely required.
Hope this has helped to clairfy some of the actual issues involved. I have worked in both the S-4 (logistics) and the S-3 (operations) sections of combat Battalions, so have a good idea what is involved in both of these fields.