Brittle and old walls can be like a chocolate box... Drilling downwards doesn't help very much, in these walls the wall plugs and screws just behave like nails: they resist almost not at all to pulling forces.
Depending on how bad it actually is, you may need to do one or more of the following (maybe even different strategies depending on where in the wall you drill. Ever hanged a kitchen on a wall built out of post-war rubble?):
- Use a smaller sized drill to make the plug a really tight fit
- reinforce the plug with quickset cement/mortar or construction glue prior to screwing the screw in
- take some matches (without heads) or toothpicks (with one pokey end removed) and hammer them in between the wall plug and the wall before screwing something in
- after drilling, use a product that gets soaked up into the wall and acts as a reinforcing primer/glue
For big holes/heavy weights there are commercial systems available that will help with glue reinforcement.
Of course you can always do the last resort and do it like people did before they invented the wall plug:
- poke a big hole into the wall, mortar in a piece of wood, screw to the wood.
Whatever you do, in these walls you have to massively derate the manufacturers specifications on load bearing abilities of the wall plugs.