Thanks to everyone's advise and the weather being too hot to do anything outside today I decided to have another crack at the drop targets.
Firstly I took out the set I had already done and just checked the tension on all the screws in the assembly and made sure everything was nice and square. I found if I over tightened the screws they didn't drop as well as backing them off just a little. I re cleaned the contact switches with alcohol and a cotton bud and applied a really light coat of 3 in 1 oil. Then I put a really tight rubber on the drop targets which pulled back further away from the targets as they seemed to be catching on the rubber a little. Success they work liked new  .
Inspired I pulled out the second set and pulled them down on the kitchen table and spent about an hour and a half cleaning them with windex, alcohol,cotton buds,steelwool and a tooth brush till every part looked new then re assembled as above.
Once again a new rubber, contact clean and a light coal of 3 in 1 and bingo another set of drop targets working like new.
Not that exciting for most of you but for me this is my greatest pinball repair to date up till now I have only really done fuses,globes rubbers,coils etc so I'm pretty happy I always find it pretty rewarding to do any little repair I can understand why you guys get so much satisfaction doing restores.
Happy New Year everyone.