That was done by my Local fish breeder, I just bought the conditioned ones and he always fed them tubifex worms.
Despite what your local seller says, you should always condition your pair yourself. The stress of being moved into a new environment alone is enough to affect the spawning willingness/capabilities. I'd also try to stay away from Tubifex- it's known for causing parasitical and bacterial infections, and bloating. I read somewhere that on large scale farms, they are fed the waste of pigs to grow more quickly. Some people may have had success with them, but in all honesty it's too high of a risk for my liking. You should be conditioning your fish on frozen bloodworms, daphnia, grindal worms, brine shrimp, or mosquito larvae. Pellets (with the exception of Pro) simply won't cut it. I believe this is the stem of the issues you're having.
Keep everyone in solitary confinement, feed them heavily several times a day, and allow your pairs to see one another only a few minutes per day. Keep this up for a week, and I guarantee you'll have more luck.
I conditioned the male by not feeding it / feeding it small amounts and my female with pellets high in protein.
You shouldn't be feeding them less- you should be feeding them much more than usual, with highly nutritious foods to get them into prime shape.
Also, are you able to answer all of these questions, right off the top of your head?
1) Are you aware of how much time caring for a spawn takes?
2) Do you know what culling is?
3) Are you prepared to cull if necessary?
4) How will you find new homes for your fry?
5) How is your breeding going to help the Betta splendens species?
6) Are you aware of the general costs involved with breeding?
7) Are you willing to ship fish if need be?
8) Do you know how to go about packing and shipping correctly?
9) How do you pick the best breeders?
10) What are the general tail types and what happens if you cross them?
11) What is conditioning?
12) How long do you condition?
13)What foods do you use to condition?
14) What items do you need for the spawning tank?
15) What temperature should the spawning tank be?
16) How do you properly introduce the breeders?
17) What do vertical bars mean?
18) How do you know when to release the female?
19) How can you tell the difference between normal and abnormal aggression?
20) When do you remove the female?
21) When do you remove the male?
22) On average, how many eggs are produced per spawn?
23) On average, how many fry usually survive to adulthood?
24) How long does it take eggs to hatch?
25) How long does it take the fry to become free swimming?
26) At what day should you introduce foods to the tank?
27) Which foods are better: live, dried, or frozen?
28) How do you get live foods?
29) What foods are appropriate for newly hatched fry? 1 month old? 2+ months?
30) How often should you feed fry?
31) At what point should you start doing water changes?
32) What is the stunting hormone?
33) How often should you do water changes?
34) How much water should you change?
35) At what point do you jar males?
36) How do you keep jarred fry warm?