I found plenty of possible sources of failure and their causes here. http://www.wetplatesupplies.com/blog/troubleshooting-the-collodion-process/

Wavey/crepe marks on surface of collodion

• Insufficient setting time for collodion before inserting into silver bath

Curved line on plate, probably lighter in colour

• Hesitation or uneven flowing of silver bath when inserting plate, worse when tray sensitising

Splash marks on film

• Interrupted or incomplete immersion of plate in silver bath

Gritty surface on plate when removing from silver bath

• Probably too many iodides in the silver bath, you need to sun your silver bath to get rid of excess contaminents
• Uneven spreading of developer over the plate surface
• Use of old developer bath or one that’s short of alcohol.
• Improper pouring technique for developer.

Black uneven marks or fog which appears upon application of the developer and spreads out across the whole surface as developing commences.

• Badly ventilated dark-room or impure acetic acid in developer

Veiling or slight fog over the entire image

• Plate has been exposed to white light either whilst in the camera or in the dark-room;
• Developer is too warm or insufficiently restrained

Fogging of fine lines

• Flare caused by too bright a light shining into the lens or overexposure.

Light grey scum forms during development

This is the fine grey scum that can be wiped away with cotton wool or washed away under flowing water. Any of the following may be the cause:

• Darkroom at a much higher temperature than the shooting area.
• Exposure to the fumes from paint turpentine, ammonia, gas, sulphides, smoke or other fumes
• PH of the silver bath is too high
• Too long a time has elapsed between sensitising the plate and developing.

Heavy tree like growth of scum starting from an edge of the plate

• These are usually known as ‘Oysters’ and are caused by dirt or dried silver within the darkslide.

Dirty or streaky marks on glass plates

• Insufficient cleaning

Grain in clear portions of the image

• Prolonged development in an attempt to correct under exposure.

Uneven density

• Uneven illumination;
• Partial drying of the collodion before immersion in the silver bath or during exposure
• Uneven coating of collodion
• Uneven distribution of developing solution.

Weak image when exposure and other factors are known

• Old collodion
• Over acidified silver bath
• Silver bath too weak

Small transparent spots or pinholes and comets of different sizes and scattered

• irregularly over the surface
• Household and chemical dust in the silver bath camera, or darkroom (Hypo is particularly bad
• Insufficient filtering of silver bath.

A regular distribution of small pin holes over the entire surface

• Silver bath is over iodised or collodion with high salt content has not been allowed to settle sufficiently.

Black spots and marks

• Dust on the plate prior to coating
• Use of reclaimed collodion without filtering
• Dust and dried collodion from the collodion bottle

Fine black lines

• Glass or aluminium not sufficiently clean
• Scratches on the plate surface.

Poured collodion cracking

• Collodion too thick
• Prolonged drying before immersion in the silver bath
• Excess ether in the collodion.

Denser portions of the image adjacent to clear areas

• New under iodized silver bath in conjunction with too weak and therefore insufficiently restrained developer solution.
• Could also be too much silver nitrate solution left on the surface prior to development, ensure plate is drained well and the rear cleaned after sensitising.

Staining

• Insufficient washing

Image appears opalescent after varnishing

• Plate was too cold before application of varnish

Image dissoves when varnishing

• Old collodion used
• Humidity too high in work area (unlikely in the UK)
• Ethanol used in varnish has been denatured with Methyl Ethyl Ketone or another agent which affects the collodion layer (unlikely unless you're mixing your own varnish with ethanol from an unknown source)

Double or blurred image

• Movement of the camera or subject during exposure. Can include vibration through the floor.

Horizontal and vertical lines not in the same plane of focus Astigmatism through the lens elements select an alternative lens.

• Poor definition even when the camera and subject are steady and free of vibration.
• Focal plane of the lens not in exact position to the film plane, stopping down the lens a little can help. Confirm darkslide is correct for the camera.