IPhone – Don’t charge iphone when plugged into macbook

This sounds like it could be an issue with the your battery's fuel guage. Lithium Ion batteries have a problem with the meter that detects how much charge is left in your device. If you don't full cycle the phone every 30 days, the meter will get worse and worse.

Full cycling involves:

1. Charge the phone to 100%
2. Do not charge your phone again until all the battery is depleted (i.e. until the phone dies on it's own).

This is recommended both by Apple:

Be sure to go through at least one charge cycle per month (charging the battery to 100% and then completely running it down).

... and by Battery University:

Batteries with fuel gauge (laptops) should be calibrated by applying a deliberate full discharge once every 30 charges. Running the pack down in the equipment does this. If ignored, the fuel gauge will become increasingly less accurate and in some cases cut off the device prematurely.

Some obvious causes:

1. Need to wait 30 to 60 seconds for the SMC to read the battery and decide what to do
2. check for pencil lead/debris on the mac end of the magsafe
3. check for pins stuck down on the magsafe cord end of things
4. The SMC needs to be reset on the Mac
5. The battery, all adapter or charging circuitry itself is faulty or detecting something that causes a pause and the mac might need service. (This is exactly what your mac will do if it needs an 85 watt adapter but you've somehow gotten your "friend's" 65 watt adapter - if your adapter isn't putting out enough power, the mac can only run and not support the CPU/GPU load as well as the charging circuitry. Similarly, if the battery is drawing more than designed, the mac will prefer to run than charge and tells you this in the not charging message.)

Due to the way batteries work, they can be out of tolerance / spec at only a specific portion of the entire voltage range. The sensors will correctly detect “good battery” until you reach that one point where the voltage drops too fast and shows aging or a problem with the cells.

Do a calibration run at your earliest convenience if your battery doesn’t continuously self regulate and calibrate. Next, think about a visit to the genius bar or other Apple Support / service desk if the problem continues. Take your wall adapter with you when you go as it can be checked with the mac together.