I actually use this method to look at negatives when I want to see a positive image for some reason. Hold the negs emulsion side up to a strong light source with a dark background behind them and you get the positive look. Works best with thin negs, which is why you probably noticed it.(posted 8215 days ago)I agree with Joe, check the edge markings 1st, that should point you in the direction of underexposure or under development. I'm guessing underexposed since you have the problem with both developers. Also, don't jump to the conclusion that they're no good. I under exposed some APX-25 one time by going the wrong way with my filter factor corrections (2 stops the wrong way... so 4 stops underexposed!), and the negs are so thin but still produce prints.
For the record, what development dilution/time/temp did you with the Ilford Plus? I use a fair bit of this combination and expose FP4+ at 100asa, then develop in Plus for 1:29 dilution for 6mins @ 20C with 3 inversions per minute.