It sounds like it's time to give your automatic shut off system a good going over.  If any of the tubes get plugged with dust the vacuum can think the bag needs changing and it won't turn on or will immediately shut off.  Does the full bag light come on?   Someone posted a step by step overview of the system with pics.  Here's a link:

http://www.vacuumland.org/cgi-bin/TD/TD-VIEWTHREAD.cgi?10846

There are a few other threads discussing the automatic system that will turn up with a search.  Once you open up the lid (unscrew from the inside) and get a look at it all, respond to this thread with another question if you need more help.

Of course, as soon as I start typing an answer, crevicetool jumps in and posts HIS answer.  Yes, duh, the switch in the lid is suspect.

As far as the bags, of course the company will tell you to use the genuine ones, but I've never had a problem with the better quality generic ones.  When you had the thing apart, did you look at the spring-loaded switch on the lid?  If the vac doesn't turn on, open the lid, turn on the power and then depress the switch.  It should power on as soon as it's depressed.  If you have to jiggle the button or depress it really hard  or repeatedly then that's where the problem lies.   By slamming the lid hard it connects but after awhile it loses contact.   Also check the little rubber funnel-shaped air tube on the upper rim of the bag compartment.  That needs to line up with and seal against the corresponding hole in the lid face to complete the automatic shutoff "air circuit." 

Problem solved, that's great.   The model 1521 ran for a number of years with only  name/color changes so they show up in sales frequently, and replacement parts are pretty easy to find.   Glad we could be helpful!

Neil

"I am having trouble turning on my Electrolux 1521."

Did you try blowing softly in his ear? hahaha!