I am 50% agree with @Paul Rutland because your primers is working with lung tissue so primer can not be an issue. But your brain RNA can be a issue. you have to check your brain cDNA with other housekeeping gene like GAPDH. You should also check your RNA quality on gel. I am 90% sure that you have issue with your brain cDNA may you have done some mistake at the time of cDNA synthesis.

All the best

Better you will take fresh tissue and then do RNA isolotaion, may be your lung sample has spoiled or contaminated. Hopefully  you will get result next time.

either your rna has degraded  or your primers are degraded are ideas worth testing. If your primers are dissolved in water they can absorb co2 from the air, become acidic and depurinate. ( borrow a set of housekeeping primers from another researcher and amplify).

If you have another rna  or cdna that has worked previously test your primers on it to make sure that they still work. If both ideas fail run a dna amplification of another primer set on gDNA to test the other aspects pf pcr..reagent,pcr machine .

I am 50% agree with @Paul Rutland because your primers is working with lung tissue so primer can not be an issue. But your brain RNA can be a issue. you have to check your brain cDNA with other housekeeping gene like GAPDH. You should also check your RNA quality on gel. I am 90% sure that you have issue with your brain cDNA may you have done some mistake at the time of cDNA synthesis.

All the best

RNA is very labile, so starting fresh is the best option, if RNA is not amplifying. 

Thank you everyone for suggestion. :))

I'll check my cDNA brain with other housekeeping gene.

I agree with what Abhishek Vats says . He has thought more carefully on the problem and I think he is correct in that the brain RNA is most likely degraded