Like the man above says you've been sold the wrong battery.
Go back and argue the toss.
Secondly make sure all leads connecting to the battery are clean, then make sure the main lead from the battery to the starter terminal is clean.
I.E. remove them and clean them with steel wool or sand...
What engine type etc is it?
What does the serv indicator say?
If it just says SERV then on the 2000-2006 models this is a MIL lamp (malfuntion indicator lamp) not a service schedule and so you'll need to get a diagnostic on it to read the code.
the battery should say 68ah if it's correct.
Even if it's a bit smaller, though not correct, being brand new it should crank the engine over. When you say it gets slower, is this after 10 secs or 2 mins?
Does the engine turn over slowly, or normally and just not fire up?
If it turns over normally confirm voltage to the plugs. You can use a multimeter as suggested but I find a 12V inspection lamp better as you can do it on your own. You'll also see how long the plugs are heated for. It should be...
Try using your jump lead to connect to the starter solenoid directly.
In effect bypassing the ignition switch.
Makes sure it's out of gear or you'll run yourself over.
This will confirm the ignition swicth wiring.
Hello
This is for the new shape Micra. It's a bit complicated so it's worth printing it off. But no diagnostic equipment is needed.
You should have 3 buttons above the radio either side of the centre display.
Let's call the one on the left of the display 'A'
The one on the right of the...