Hickok Tube Tester Help Needed

I just had an unusual thing happen with my tester.  after testing a few tubes (12AX7s etc.) and leaving the tester on for an hour or so, I tested a 6550 and the needle barely moved.  Before blaming the tube, I tested a known new KT88 with same results.  That was yesterday.  This morning, I tried it with same results.  Thinking it might be the socket, I tried a 5Y3 which tested very good.  
Now I am lost.  Might be something to do with the switch settings, etc.  I am no expert on tube testers, so if anyone has an idea, please let me know.

What model Hickok Tube Tester?

Does the 6550 filament lite up?

Sorry for the goof, and thanks for both responses. It is a 600A and yes, the filaments do light up as strongly as normal. I have looked inside ans there are no loose or disconnected wires to the octal socket. As recently as a few days ago, I had tested some 6550s with no problems. Yes, I did use a spray cleaner on the socket and on several of the rotary switches. Bob, I think you may be on the right track, since other tubes test OK.

Does anyone know of a good tube tester repair person?  I have tried to move the octal tube socket contacts (it is an old type with two "fingers" at 180 degrees from each other) with no help.  If I move the "English" pot I can get a good reading at a setting way below the correct one.  Also, I just tested 3 new 12AX7s with correct settings and the needle pegged on the right side - looks like it is all "haywire" now.

kygeezer wrote:Yes, I did use a spray cleaner on the socket and on several of the rotary switches.  Bob, I think you may be on the right track, since other tubes test OK.

Just a word of warning for those that don't know - you have to be very careful using contact cleaner on the rotary switches in a tester. A lot of contact cleaners are oil based and/or conductive. If these types of cleaners are used on the rotary switches they can soak in to the wafers causing capacitance and leakage, essentially permanently destroying the switch as there's typically no way to remove the cleaner once it soaks in. Even with alcohol, I've also heard of small metal particles from the switch contacts getting impregnated in switch wafers when too much cleaner is used, also causing leakage and destroying the switch.

As most tester switches use silver plated contacts, typically only minimal cleaning is required as silver tarnish is more conductive than clean silver.