You really have 2 threads here. 1 for pocket lifting and 1 for yellowing.
Yellowink (as it is pronounced in youtah) can be caused by 3 things:
- contamination
- incompatibility
- UV light
contamination
This is when something is there that shouldn't be. As previously mentioned, brush contamination is usually the culprit.
If the yellowing seems to get less as you work on a set, then you are likely looking at brush contamination straight between the eyes.
This is usually caused by left over monomer partially polymerising in your brush (i.e. the ferule) and getting flushed out during your application.
incompatibility
This is when you are using products that are...um... incompatible.
Good examples of this is: Primer getting on existing product (designed for the nail plate only), solvents on product (i.e. Nail Fresh on existing product), Intermixing systems (i.e. using Brand A monomer with brand B polymer)
UV light
UV light can have a really lame affect on enhancements. UV light strips out parts of the enhancement and force it to reflect yellow light. UV light can also slow bake UV cured enhancements, causing them to become brittle and yellow. Most L&p enhancements will contain UV absorbers and optical brightners to counter this effect. Avoid faster setting powders as this can worsen the situation (fast it sets... faster it cures... faster it ages)
Yellowing is usually a sign that something is wrong. It is a symptom of enhancement aging and will often be combined with brittleness.
If your enhancement yellows within a few minutes, the reason is usually contamination. If its hours later, its usually incomaptability, if its later than that... its usually UV light.
Pocket lifting sucks almost worse than yellowing.
Pocket lifting is when there is an area of lifting in zone 2 (the body of the enhancement)... it looks like a big ol bubble.
This is usually caused by a combination of 2 things:
Excessive wet ratio and thick application.
The wetter the mix, the more the shrinkage. The thicker the application, the greater the effect of the shrinkage.
Hope this helps