Sticky plants

[coccinelle]

My houseplants in the dining-room have been giving off a sticky substance since last year. I bought a fungicide/ insecticide treatment for indoor plants and used it twice but it hasn't solved the problem. The back of the leaves have got red spots on so know this is the problem but why doesn't the treatment work? I'm considering chucking the whole lot and starting again with new plants (pity cos some are 15 years old). What are the risks of this problem starting again?
I'm more of an outside gardener than inside and this is proving its point!

[Hawkwalk]

Most probably scale insect. See link for best treatments.

http://www.rhs.org.uk/Advice/profiles0801/scal e.asp

you could also try wiping off using a mild fairy liquid solution and a cloth. This may not get them all but should reduce the numbers and remove the sticky residue. then treat as mentioned in the link.