truly random shortmers should anneal almost everywhere but being short it will be possible for them not to anneal if the surrounding room temperature is higher than the annealing temperature of the 6mer. many cdna kits use 9 mers

Are you only using random hexamers? Some kits also use poly T tails to complement random hexamer priming.

• No Random hexamers as there name implies are a sufficienlty randon population that for an mRNA transcript of say 2kb they will anneal at least 5 times
• If your experiencing problems with making cDNA, then unless you are using an old aliquot of Random hexamers that have been freeze thawed multiple time - in which case they might not work - then the solution to your problem lies else where
• Thus, I would check the quality of your RNA (260/280 = 1.8 to 2.2; 260/230 > 1.0) use 1ug of high quality total RNA and reverse transcribe for > 1hour and use a fresh aliquot of random hexamers (or if bundled into your qPCR mix, fresh mix)
• If incidentally you are specifically trying to random prime mRNA as opposed total RNA then problems can arise with insufficient amounts of specific mRNA which is technically more difficult to isolate reliably than total RNA