OK Jennylyn let's sort out this spark plug issue mystery!
With all plugs in place if you turn the ignition key to Start does the engine turn nicely over or does it struggle to move?
If you remove any plug does the engine now turn over faster and start? If so it would seem that there is not enough power in the Starter Motor to overcome the resistance you get from the engine when it is compressing on all cylinders.
If this is the case you should first get your Battery Load Tested to see if it is capable of supplying enough current to turn the engine over.
If the battery tests good or you get a new battery and you still have the problem then either the Starter Motor is bad or there is a voltage drop on the starter motor or a voltage drop on the electronic modules.
So with a good fully charged battery in place, check that both the battery connector leads are clean and tight. Make sure that where the negative lead goes to chassis that the connection is clean and tight.
There are two straps at the rear of the cylinder heads which ground the engine to chassis...make sure these two grounding straps are clean and tight both ends.
You need to get a Multimeter if you do not already have one...a cheap digital multi meter will do...I never travel without one in my Jeep!
Turn the meter to DC Volts and with the ignition OFF place the meter leads across the battery positive and negative leads.
The voltage should be about 12.5 volts or higher.
With the meter leads still in place...as you turn ignition to Start and the engine starts turning over...the voltage should not drop much below about 10 volts.
When the engine is running...no matter how you got it to run...the voltage on the battery should be around 13.4 volts.
If the voltage while turning the engine over is below 10 volts then the Electronic Modules such as the BCM, PCM and TCM may not operate properly so it will not fire!
If you have a good battery fitted...try get a thick cable such as the two thick cables on a Jumper Lead Set.
Take say the Black cable of the Jumper Lead Set and clamp one end on the Negative Battery terminal.
Clamp the other end of this Black Jumper lead onto the metal body of the Starter Motor...this ensures that the grounding from the battery to the chassis/engine will now definitely be good.
If engine now turns over nicely and fires up then your existing Grounds are bad ie. the Negative Battery lead going to chassis is not making good contact or the two grounding straps at the rear of motor are not making good contact.
Or your new Starter Motor is faulty.