Veers in one direction
Difficult to steer
Most likely cause is that the front wheel is not correctly mounted in the front forks, so it is not pointing straight forwards.
To solve this:
Important A pushchair will naturally veer if the handle is held off centre (you are in effect steering it) or due to the camber on a road surface. If you are running with the pushchair holding the hnadle with one handoff centre then you can make fine adjustments to the front forks to 'out-tune' this steering.
If the occupant does not sit normally in the seat but curls forwards and to one side then this will cause the pushchair to veer and be very heavy to steer.
Veering will be most apparent on a smooth surface it is commonly masked when the pushchair is used on the rougher surfaces it is designed for.
Flat Tyre
Puncture
The wheels have super thick heavy duty thorn resistant inner tubes which are far superior to ordinary bicycle inner tubes. They significantly reduce punctures but they are still possible.
You can either patch repair the puncture, just as you would with a bike, or replace the inner tube.
Whichever option you choose we recommend that before you remove the inner tube you mark on the tyre the position of the valve. Then when you test the inner tube to find the puncture if the hole is say 6" from the valve you will know where on the tyre to look for and remove the cause of the puncture, e.g. a thorn.
If you have the work done by a bike shop then make sure you get them to remove the cause of the puncture, otherwise they will get repeat business.
If you choose to replace the inner tube DO NOT fit standard bike inner tubes because these are much thinner and will suffer regular punctures.
Valve Chopped Off
It is important to maintain tyre pressure at around 40psi especially if the occupant is heavy. If tyre pressure falls too low then the wheel can rotate independent of the tyre and the inner tube valve is chopped off.
This is obviously not repairable, the inner tube will need to be replaced (see information above).