Microwave not heating things properly

[Jemster]

I've noticed our microwave is not heating things like it used to, they are taking much longer. Anyone know why this might be and if it's fixable? Thanks

Oh gosh maybe thats it. I do wipe it down but maybe it needs more of a deep clean.

Is there anything on top, like has someone left a magazine on it, or too close the the side? This affected mine, think I had cook books beside it or summat (was ages ago so I can't remember exactly)