It is normal for the battery to lose some capacity while sitting. And the more the engine is cranked without starting, the lower the battery gets. All normal. Jump-start compensates for low battery. That's all. It doesn't help start the car, all it does is transfer enough battery to turn the engine over at normal speed. If the engine is turning too slow it usually won't start

So why doesn't it start right up? Usually because of a lack of gasoline. Pump the pedal 2 or 3 times to squirt gas into the engine, then twist the key to START. However if the engine has been sitting for a while, the carburetor will be empty and will need to be cranked for a while until the fuel pump fills the carburetor. After that, 2 or 3 pumps will do the job

The second most commmon problem is spark. If you engine still has the original-type points distributor, the points may be corroded and so not starting. Per the owners manual they are supposed to be replaced every 12 months