Why does my blade gets stuck in the wood while cutting?

Recently I noticed it's began sticking (stop cutting) whilst cutting and I have to raise the blade and have several attempts to finish the cut. I am only cutting pine wood.

It sounds like the blade is either extremely dirty, or very dull, or both. A new blade is probably the fix you need, but you could try cleaning the one you have first to see if that helps. Pine is notorious for the pitch it contains, which builds up on saw blade teeth over time.

At first I thought the the blade sounded like it may be loose so I tried tightening it but no joy.

It should be easy to tell whether the blade is slipping or not: if the blade is loose, then you'll hear the motor continue to run even as the blade is stuck. If that's happening, then you definitely do need to tighten the blade.

More likely, though, the blade is stopping because the motor can't turn it due to dull and/or dirty teeth. Other, less likely possibilities are that the motor is somehow damaged and no longer provides the same power, or that not enough current is being supplied to the motor. If you're using an extension cord to connect the saw, try removing that and plugging the saw directly into the wall.

Another possibility (which I don't think is likely, but I'm adding here for completeness) is that the kerf is closing after the cut and the material is binding the blade.

The main cause of this would be that your workpiece is improperly supported. If you have auxiliary supports outside the saw itself make sure that they are level with the saw. If they are higher as the cut weakens the board it will "hinge" shut on the blade, pinching it until it stops.