did you see any smear or just a blank background?

did you use DNAse??

how many ug you loaded for the detection?

Typically you would want to use formaldehyde agarose gels for this - and would need to load about 5 to 10 ug RNA per lane to get a readable signal.  Perhaps you are not starting with enough RNA.  How many PBMCs did you extract this way - and what was the volume of nuclease-free water added back to the final pellet?  2 to 10-million cells per 1 mL of TRIzol is what you need to extract -- and adding back 70 to 100 uL of nuclease-free water to the pellet would give a good starting point, but, even then, you may not have enough RNA to successfully perform an RNA formaldehyde/agarose gel.  Bioanalyzer 2100 would be a better option for this.  

I extract RNA from bacterial cells using Trizol and run them on 2% agarose gel (in 1xTAE) and I am able to see the rRNA bands. I guess you should also try a higher percentage of gel and don't run it for longer duration as the faint bands will not be visible.

I will recommend that you first measure the concentration of your extracted total RNA on a nanodrop before resolving a known concentration ( about 1ug) on 1% agarose-formaldehyde gel.

If you have a bioanalyzer available to your lab, it will be worthwhile to use it to check if your total RNA is degraded or not. The positions of the rRNAs of interested can be identified from the peaks shown in the graph.

There are several threads on ResearchGate with similar questions.    (Use the dropdown menu in the Search box to select "questions" and then type in your search key words.)

The first of these two links is particularly useful:

https://www.researchgate.net/post/Can_anyone_help_me_with_gel_electrophoresis_of_total_RNA

https://www.researchgate.net/post/How_can_I_check_the_RNA_integrity_and_know_if_the_observed_bands_are_from_DNA_contamination_or_not/1

Any advice on RNA electrophoresis: poorly stained ladder and controls? - ResearchGate. Available from: https://www.researchgate.net/post/Any_advice_on_RNA_electrophoresis_poorly_stained_ladder_and_controls [accessed Sep 21, 2015].

1% agarose usually for DNA running. maybe you can run in 1.5-2% gel agarose and tretament your RNA with DNAse.

Have you seen a RNA-pellet before resuspending? If not, there's a chance you've lost your RNA during a washing step or so...

For our gels we use 1% Agarose, melted in SB-Buffer dyed with EtBr. We run it in SB-Buffer, 15min, 180V, 400mA. Normally I get clear bands (or smears, if something went wrong...).

Good Luck!

Hi if you saw nothing as in comments above then increase the amounts of RNA sample you are loading - try double. Also Id use a 2% agarose gel as this will separate the 18s and 28s better - the bands should look discrete and if the top band is bigger than the lower it shows your RNA has good integrity.

How long are you running the gels for also and are you careful to RNase treat the gel buffer, glassware surfaces etc. Your RNA might just be degrading WHILE you are running the gel!!

I used to RNAse treat the buffer, use 2% gel and run the gel v fast and quick  (100v for a small gel) run it for 20 mins or so just until the loading buffer is about 3cm below the wells.

Best wishes and good luck!